It is far better to have a separate “breeding” tank for spawning, although it is feasible for spawning to occur in the main aquarium. If you do not separate the breeding pair from the main aquarium you are then in a position where you cannot condition the pair properly, the breeding fish will cause havoc as they prepare and defend their spawning site, especially Cichlids, and you will have to be constantly vigilant, watching for the eggs or the young to delivered, it will be an almost impossible task. The exception to this is if your main aquarium is a species tank with just the mated pair in it, this can then be used as a permanent spawning set-up.
The size of the breeding tank will vary depending on the species of fish that you have. This will be made clearer on the particular species pages. If your adult fish are going to spend long periods in the breeding tank then it will need some sort of filtration. Large power filters that cause strong currents are not recommended, the current will disturb bubblenests and the intakes could well suck up any small fry when they are born. In most cases a simple bubble-up sponge filter powered from an airpump will be quite sufficient. This will be adequate filtration for one pair of fish and gentle enough so as not to disturb eggs or fry.
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A suitable alternative to live plants, or imitation ones come to that, is the spawning mop. Spawning mops are more often home made, in fact you will rarely see them on sale at aquatic stores, which seems odd considering their usefulness.
They are best made from green nylon wool; green being a more natural colour. To make the mop you’ll need a piece of stiff card which measures around 6 inches by 9 inches (15cms by 23cms), this way you can make two different sizes of mop.
Wrap the wool around the narrow side of the card, about 30 times. When you’ve finished wrapping, cut the wool off level with the card. Take a separate piece of wool about 9 inches (23cms) long and thread it under the strands of wool at the edge of the card, tie this in a knot so that it leaves you with two equal lengths, as well as tying all the strands together. Using a pair of scissors cut through the strands of wool at the end opposite to the knot, you then have your finished mop. The two loose ends that you tied the strands together with can be used for securing it above the waters surface, or maybe to a cork so that it floats.
To make a larger mop use the same method, but use the longer ends of the card, and wrap the wool around it about 40 times. In either case you can experiment yourself with different sizes that suit your requirement. The spawning mops can be washed in warm, but not boiling water, do not use detergents. Before spawning mops are re-used they must first dry out completely, and then be washed in warm water as before.
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As I mentioned at the beginning you could be faced with hundreds of fish, and if you only have a minimum of tank space, say one or two rearing tanks, then its unlikely that you will be able to rear more than about 20-40 fry to adulthood. If you have all of the fry crowded into small tanks, the time will come when they start to become stunted. If you let this happen all the fish will suffer and may even die. If you are fortunate enough to have plenty of space, and can provide enough tanks, then you will be able to split the fry up and be able to rear most of them. You maybe thinking to yourself why not just get a large tank and put them all in that? It’s not quite as simple as that!
An important point to remember is that small fry seem to do well in small rearing tanks, this way it is easier for them to find food. Moving them to larger tanks as they grow ultimately appears to produce bigger and better fish. It’s also important to remember that, as certain species mature they do not appreciate being in large groups of their own kind, Siamese fighting males in particular will actually kill one another, and Cichlids for instance, will fight as they start looking for mates.
Try to move the young fishes on to other tanks if you can, if slitting up the fry into several tanks is not an alternative for you, then as distressing as it may be, the only option is to cull the surplus fry, keeping the best of the young for future breeding stock. You may be wondering what is the best way to do this.
It is probably best to cull the fry when they are very young, and to let nature takes its course by letting the young fish swim with a tank of adult fishes. An alternative to that would be to put them to sleep with an anaesthetic, but I don’t know if the anaesthetic can be obtained from a vet, or whether he/she would have to come out to do it. You would need to approach your vet about this.