Often times aquarists assume that a particular species of fish has to be kept at a certain temperature in order to do well. We've found a couple good reasons to deviate from this practice. You too may find this useful in managing your breeding operation. First, you have to make sure to keep them within their temperature tolerance range. Most would be very surprised to see how wide this range can be when care is correct.
Keeping fish in the lower range of their temperature tolerance will allow you to stop a pair from spawning and save it for a later time when you may need the spawns. Keep them warm and they will keep on spawning even if you can't use the eggs. We use this technique to keep back-up pairs in prime breeding condition and ready to go if we need them. They seem to pickup right where they left off before we cooled them down.
The life span of the fish can also be greatly increased when kept in the low end of the temperature tolerance range. We've experienced life spans that are 3 and 4 times what you would normally expect, when fish have been kept cool for long term. Fish grow their whole lives, even though growth slows down quite a bit as they age. Fish that live 3 times as long as one kept warm, will eventually get larger. So, even though it will take longer to get them to a large size, the maximum size is usually increased. Like most hatcheries where the room is heated, we have tanks on the bottom rows and in the corners, that are naturally quite a bit cooler. We put extra adults in those. The fast growing juveniles that we want to grow up quickly are kept in the warmest tanks and breeding pairs are usually in the mid-range tanks. This sorting allows us the best use of the tanks we have.
You have to be careful though. At lower temps, their food requirements are drastically reduced. Overfeeding in this low range is very common and will wreak havoc with their health. It's not really an immune system problem, but a husbandry mistake that leave most people thinking that fish should not be kept cool. As an example, we've had angelfish in the upper 60's and lower 70's - long term with no trouble. We even once discovered a spawn with a pair at 68 F. However, at this temp they barely need to eat. Their metabolism is surprisingly low. They can easily go two to three days between very light feedings. You may want to experiment with these techniques, just be careful and very observant.
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