I have a 125 gallon tank; temp is 78-80 degrees Fahrenheit; ammonia and nitrites are 0 and nitrates are around 10. The substrate is gravel; I have been unable to afford a good light for this tank (they are expensive!) to be able to put in good plants, but I am saving up the money for that as well. The fish in it are ONLY mollies.
Lately, many of my "tweens" (between fry age and bearing age) have been falling ill to a mysterious disease. They sometimes get skinny first, but most of the time they do not. Then they start whirling and get a crooked spine. At that point, I have to euthanize them because it is inhumane to watch it.
The weird part about this mystery illness is that it will hit the fish in a matter of hours. One minute the molly is swimming around happily eating/pooping/playing, and the next minute he or she is whirling with the crooked spine. It's become very heartbreaking to watch. Another weird thing about this mystery illness is that it only affects "tweens"; my adult fish are not bothered by it at all.
I have treated the tank with antibiotics thinking there was some sort of internal bacterial infection because one of the fish developed pop eye (none of the others did). That did not stop this. I started adding more salt with the water changes. That has not stopped the whirling. I have tried formaline in case this is a parasitic infection; no change. On a daily basis, I am pulling out dead or almost dead fish from this tank.
A few possible scenarios have crossed my mind: Livebearers are prone to inbreeding. Could this be some sort of inbred birth defect?
Another possible scenario that I thought about was this: several months ago I treated the tank for calanus worms and everyone healed up fine; there are no more worms. I treated with Levamisole (recommended for treating these worms) that I got from our local feed and seed store. I measured it down precisely with a milligram scale and dosed the tank accordingly. These current "tweens" may or may not have been fry at that time. It is hard to keep track because some of the current "tweens" were in the fry tank which I also treated for the worms as I was quarantining them at the same time of the worms outbreak. Could that have affected them severely? Could the levamisole affected any pregnant females that were carrying fry so that the fry could have birth defects?
A final scenario is that I just have not yet found the right combination of medications to give to the tank. I used doxycycline because the water here is very hard (i.e. high in natural calcium) and because tetracycline does not do as well in harder water then the derivative, doxycycline, is used in its place.
Any help or thoughts or suggestions would be greatly appreciated! I love these mollies like they were part of my family, and they are! I have included my e-mail address. Please feel free to email (please no spam) or reply to the post if you can think of anything. Thank firstname.lastname@example.org