How do I set up this rearing tank? 

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Neale Monks advises a reader on protecting his newborns in a rearing tank.

Q: My Guppies have recently bred and right now they are in a breeding trap floating in the tank. I have about 30 fry in there of different sizes, and every couple of days I’ll spot another one or two hiding near the bottom of the tank and net them out to add to the trap.

I want to set up a rearing tank for them, as this trap is getting too crowded now, and I’m scared of releasing them back in to the tank even when they get a bit bigger, as I have a large Angelfish that keeps investigating them inside the trap. I’m sure he has started to think of them as food in waiting!

Can I set something up just using the water from my existing tank, or will I still have to cycle it before I can add them to it? And advice appreciated.

KEVIN SCOTT, VIA EMAIL

A: Neale says: Angelfish will enthusiastically consume livebearer fry, and adult Angels can eat small Neon tetras, so you’re very wise in keeping newborn Guppies away from the other members of your community tank. If the tank only contained adult Guppies and gentle, bottom-feeding species such as Corydoras or Kuhli loaches, the fry would be safe enough returned to the main tank after maybe 3-4 weeks, depending on how fast they grow. 

But Angelfish, and any other opportunistic predators, will view anything bite-sized as potential prey, so you’d need to keep the babies away for longer. Indeed, I’d imagine the Guppies would need to be kept apart from Angels for 2-3 months before it would be worth risking them, and even then the smaller males would very likely excite the wrong sort of interest!

Newborns, or even specimens a few weeks old, will be producing precious little ammonia, so a tank upwards of 45 l would dilute their wastes pretty well, especially on top of frequent water changes. Indeed, the frequency of water changes is the key to success when it comes to rearing baby fish, with high nitrate levels seemingly linked to elevated mortality rates and slower growth rates. 

I’d also do my best to ‘clone’ the existing filter so that bacteria are taken across from the community tank to the rearing tank. This can be as simple as taking some ceramic noodles or filter wool from your mature filter and stuffing them into whatever new filter you elect to use. Floating plants are another way to speed up the operation, and species with feathery roots, such as Indian Fern, are absolutely ideal for this. 

Some aquarists in fact prefer to leave the bottom of breeding tanks clear so that they can be more easily cleaned, and instead rely on floating plants to provide the shelter small fish need. Couple this with an air-powered sponge filter and you’ll have a cheap but very reliable system for rearing livebearer fry.

Sadly, adding water from a mature tank to a new tank doesn’t do much to speed up the cycling process. It’s probably better than nothing, but very few of the helpful filter bacteria live in the water column; instead, they attach themselves to solid surfaces where oxygen levels are high. 

That’s why you find them on the roots of floating plants, on the substrate, and of course on the types of media used in biological filters.