Breeding: Spawning your fish on a Mop

Breeding: Spawning your fish on a Mop
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Breeding: Spawning your fish on a Mop



Ever on the look-out for breeding tips from the world’s breeding gurus (and preferably low-tech gurus),  I recently came upon another gem offered by Ted Judy. In his article titled “Yarn Is Easy” he begins to explain with a little story in his typical dry-humour style:

The store clerk says, “You buy a lot of green yarn.”

To which I reply, “I like the color green.”


“A whole 10 bundles worth of green?” she asks.

“Sometimes I buy dark blue.  But only when you are out of green.”

Miss Nosy presses, “You buy all the green.  I think you are the only person who buys it.  Why so much yarn?”

“Mops,” I replied with a look that suggested it is perfectly normal to buy 10 large packages of green acrylic yarn to make mops.


“You make your own mops?”


“There are better mops in house-wares.  Thicker.  Made of cotton.  They hold water better.”


“But those are white.  And cotton rots.  And they’re way too big.”

“Too big for what?”

“Mops.  And it is too hard to see eggs in a white mop.”


She stares at me for a moment, and turns away as she concludes, “Have a nice day, sir.”

(Appreciating this kind of humour, I was very tempted to title this article “Go Buy a Mop”)

Ted continues as follows:


Aquatic plants are the egg depositories, nurseries, and fry-food larders in most of the ecosystems from which most of our aquarium fish originate.  In some places the water is shallow and in a sunny place, permitting dense growths of plants all through the habitat.  Other places have deeper or fast flowing water that limits the vegetative cover to the shoreline.  And there are many places, such as deep under the canopies of forests, where there is not enough light to grow plants well.  And yet, even in these light-starved locations there will be enough plant growth to provide fish a place to spawn and the fry a place to start life.

Except in my fish room.  I am notoriously bad at growing aquatic plants.  I have a few forlorn clumps of java moss here, a bit of various water weed there and some ever-present duckweed on one or two tanks.  I try to grow plants.  I rarely return from an auction without a few bags of vegetation.  I will always buy guppy grass Najas sp., overpay for Java moss, and bid competitively for frog bit and water lettuce; but no matter how much I bring home, I am in need of more within a couple months.  So I dutifully buy the plants and they dependably fall apart into lovely detritus (decaying plants that many fish love to eat).  Not a perfect system, but it seems to work for me.

Except that adult fish need plants to lay eggs in and their fry need a place to hide and forage for food.  Since I cannot count on having a dense growth of live plants to provide these places, I have learned that the next best thing is yarn.  Yarn mops are the staple for killifish and rainbow-fish breeders, but yarn can be used by any fish that lays its eggs in plants.  The trick is in how the yarn is presented, and to get the presentation right it is important understand how the fish use plants.


Egg Placers

Some fish are very specific about where they lay their eggs.  They do not lay many at one time and will carefully choose where to put them.  Killifish and rainbowfish are good examples of these egg placers.  The eggs are adhesive, and they stick where they are laid.  Males will stake out territories in an area where the females are likely to want to lay their eggs.  This might be a particularly dense growth of plants in nature, but in my fish room it is a strategically placed mop of green yarn.  Egg placers can be picky.  Some species prefer to lay their eggs in the middle of the mop, and will work very hard to get themselves as deep into the yarn as possible.  Other species will barely enter the mop at all, and lay their eggs in plain view on the outside of the bundle of yarn.  Sometimes the place where a female lays her eggs is different between individuals.  I have a colony of pygmy rainbowfish Melanotaenia pygmaea, and some of the females choose the very top of the mop and others prefer the very bottom.

Breeding: Spawning your fish on a Mop Ted mop 1

Egg placers like rainbow fish and killifish will readily use a yarn mop

I was surprised to learn that some species of Corydoras catfish will deposit eggs in a yarn mop.  Cories are one of the most deliberate of all egg placers.  The female will carry a few eggs between her ventral fins, clean a site with her barbels and then meticulously place her very sticky eggs on that spot.  The first Cory I worked with that used a mop was C. panda, which is a relatively easy cory catfish to breed and is readily available in the hobby.  Condition them with a lot of high quality food, including live foods such as black or white worms, and give them frequent water changes with cool, clean water of medium hardness and a neutral pH.  Leave a mop hanging from the surface to the bottom in the tank near the current of the filter.  The panda cories will also lay eggs on the sides of the tank, so when eggs appear on the glass there are probably also eggs in the mop.  I remove the entire mop with eggs to a hatching tank and add quite a bit of methylene blue to the water (2 to 3 drops per gallon), and I have found that there is a much greater hatch and survival rate than without the medication.

Egg Scatterers

Almost all of the characins and cyprinids (tetras, barbs, danios, etc.) like to scatter their eggs over plants.  Egg scatterers fall into one of two categories that I use to describe them.  The first are the true scatterers.  The males of these species chase the females all over the tank until the females give up trying to get away and just dump their eggs.  I do not know if egg release is a stress response or not, but I have had very gravid congotetra Phenacogrammus interruptus females unload a lot of eggs while being chased around the tank with a net (which does not leave many eggs for spawning later, so net carefully).  The pairs or spawning groups will dash about all over the tank dropping eggs and milt wherever they go.  The fish in the group that are not spawning will usually scurry around after the spawning fish eating the eggs just as fast as they are laid.  This is when the yarn is useful.  In nature the eggs would fall into dense thickets of plants, but in my tanks the eggs fall into yarn. Lots of yarn!  When I use a 10-gallon tank for spawning an egg scattering species I will place a layer of tangled up yarn in the bottom that covers the entire surface area to a depth of 4 to 6 inches.  The fish will spawn over the yarn, into which the eggs fall and are therefore harder for the other fish to find.

Breeding: Spawning your fish on a Mop Ted mop 2

Egg scatterers that need a large tank will spawn over a yarn egg trap if it looks as if it were grass.


The second group is the secretive egg scatterers.  These fish are a bit like the egg placers, but the eggs are not adhesive so they fall into the yarn rather than stick to it.  Most of these species are small and shy.  Some spawn every day and only lay a few eggs at a time.  A good example of this type of fish is the celestial pearl danio Danio margaritatus, which is also called the galaxy rasbora.  These shy little fish are easy to keep and breed in a very small tank.  I use a 2.5-gallon aquarium with a small sponge filter and a dense layer of yarn.  The fish are very shy and the yarn provides excellent cover for them.  I have found that a pair is best in a tank that small because the other fish will find and eat the eggs.  I leave the pair in the tank for a week and then remove the pair.  After a few days I carefully lift up the yarn and use a flashlight to look for fry.  The danios rarely disappoint.

Breeding: Spawning your fish on a Mop Ted mop 3

Small scatterers, like these ember tetras, like to bury their eggs deep in dense plants… or yarn!


I also use yarn to build egg traps for larger tetras, barbs, or danios to spawn over in a larger tank.  The trap is basically a plastic box with a wide-mesh top into which 2 – 3 inch pieces of yarn are woven.  I place the box in the larger tank with the fish out in the open.  When the fish spawn they usually make scattering runs over and through the yarn sticking up from the trap.  The eggs filter through the mesh and end up safe and sound inside the plastic box.  This system is especially useful for species that take a while to settle into a spawning tank, so moving them from conditioning tank to spawning tank just upsets them.

Breeding: Spawning your fish on a Mop Ted mop 4

Yarn can also be used over a false bottom that the eggs will fall through, protecting them from being eaten.

Traps are also good to use when the species is a very sporadic spawner.  I keep traps in all the tanks in which I have larger African tetras.  Species like the yellowtail congotetra Alestopetersius caudalis, the African red-eye tetra Arnoldichthys spilopterus, or the long-fin Alestes tetra Brycinus longipinnis.  It is hard for me to predict when these fish will spawn, but I dutifully check the trap every day and find eggs in there once or twice each month.


I also use yarn to provide a place for livebearing fish to release their fry.  Some livebearers are notorious for eating their fry almost as soon as they are free swimming.  Guppy traps are useful tools, but try using one for a 4-inch Ilyodon furcidens or other large and powerful Goodeid livebearer.  Even some of the larger female swordtails would challenge the average guppy trap.  I prefer to use a 2.5-gallon aquarium with a lot of yarn.  I use large mops that drape a along the bottom for this purpose.  The mops are thick and take up a lot of space in the water column, but leave enough room in the upper half of the tank for the female livebearer to swim freely.  The bottoms of the mops lie in a heap on the bottom.  When the fry are born they can drop into the yarn at the bottom to hide.

I move the females to a birthing tank when they are three to six days away from giving birth.  How do I know when that is?  I use a calendar.  The gestation periods for most livebearers are well-known, and a little research on the Internet is all that is usually needed to get that information.  Like all of my breeding fish, I like to condition females and males separately, and then put them together when they are fat and happy.  Most livebearers are very efficient when it comes to impregnation, so I mark the calendar on the day I put the males in with the females I want to breed.  I leave them together until the females are noticeably pregnant and then move them back to the conditioning tank; or I leave the females in with the male until the calendar tells me the fry could be born in a few days.  Once the female goes into the birthing tank I check for babies several times a day.  Once the fry are born I try not to leave the female in with them any longer than necessary.  I am also very careful to make sure that a female in the birthing tank is well fed.  I figure that a hungry female is more likely to eat her fry, but I have no evidence to support or refute that claim.  Seems like common sense to me.

The Ease of Yarn

Yarn does not die.  It will stay just as lush and vibrant regardless of the light, pH, temperature or hardness.  It comes in a rainbow of colors (though I prefer dark forest green).  And, best of all, a fish does not care if it has yarn or plants, so long as it has a place to lay eggs and hide its babies.  Yarn is easy.

So there you have it, spawning on a mop explained in the nicest possible way. If you would like to try this method, here are the instructions for making your own mops:


How to Make a Spawning Mop

  1. Start with a skein of dark acrylic yarn. Your fish will not care about the exact colour, but a dark yarn will make it easier for you to notice when eggs have been laid.
  1. The yarn must be able to tolerate a few minutes of boiling, because placing the yarn in boiling water is a great way of killing off any malicious microorganisms. Boiling and carefully rinsing the yarn afterwards will also remove any residue dye. In addition to this, a well soaked skein will be more prone to sinking instead of floating.
  1. Take a hardcover book of which the length is roughly the same as the height of the aquarium in which you intend to use your spawning mop.
  1. Cut off a small strand of yarn and place it across the width of your book, before you start to wind the rest of the yarn around the length of the book.
  1. When you have wound your yarn around the book about one hundred times(a big mop), you can cut of the yarn from the remaining skein.
  1. Take the strand of yarn that you previously placed across the width of the book and tie a knot somewhere in the middle.
  1. Flip the book and use a scissor to carefully cut the yarn in the middle to create a mop.
  1. Take another strand of yarn and tie it to the top of the spawning mop.

Pro tip: If you want your spawning mop to float around in the aquarium, tie it to a piece of cork.

Breeding: Spawning your fish on a Mop wind the yarn

Wind your yarn around a book, many times.

Breeding: Spawning your fish on a Mop Tie the yarn1

Tie off your yarn with the piece you placed first. Cut through all the yarn at the opposite end opposite end

Breeding: Spawning your fish on a Mop Finished Mop

Fold your yarn over at the tied centre, then tie another piece around the bundle, tassel-style. Your mop is now ready to be boiled. yarn