My fry tank water is warm to touch in the morning where I live. The water coming out of the faucet is cold in comparison. I also had a lot of dead fry in the tank that I keep clean, until I made the adjustment. I am pretty much sure it is the water changes that I do every morning that made them die. I previously used the water hose to gently flush what ever was on the bottom to the back end of my 300 gal tank where the water exits through my Hydra Pro #5 sponge filters on two-one inch pvc into a trash bin bio filter, and pumped backed into the tank by a 900 gal per hour submersible mag pump. The ventriled return is submerged into the water about 3/4 of the way down and is slanted to the front end where the water is dispersed evenly to both sides. The middle to back end of the tank is calm. There is absolutely no ammonia or nitrite.
The adjustment I made is I don't use the water hose to flush the bottom anymore. I use a turkey baser to flush the bottom debri to the back end. Also, I reduced the speed in which the tab water refills the tank, giving them time to adjust to the change in temperature. This made a big difference.
Oh by the way, I have another 600 gal tank with 300 gal of water that I don't clean every morning and the fry in that tank is the same age, but 4 times bigger. The filter set up is different, it has a flow through lava rock planter, 4 submersible sponge filter pumps and a 2000 gal per hour external pump hooked up to an ultima 2000 filter which circulates the water in a 100 gallon tank which returns the water from two of the submersible sponge filter pumps back into the tank through a one inch pvc. There is absolutely no ammonia or nitrite.
In conclusion, my fry deaths were caused by my Obsessive–compulsive disorder.
(OCD) is a mental disorder in which a person feels the need to perform certain routines repeatedly (called "compulsions"), or has certain thoughts repeatedly (called "obsessions").[1] The person is unable to control either the thoughts or activities for more than a short period of time.[1