We are new to this forum so firstly I'd like to say hi. Secondly, I'd like to ask for help.
Recently we were given a fish tank. We decided to switch from the one we had and make this our new one. It's approximately 20 gallons. The tank we had at first was fully settled and the fish we had in it were doing great. We fully cycled the new tank before the big move and after about a week added a few more fish to increase our numbers. Everything was going swimmingly (excuse the pun).
The tank at that point consisted of:
2 African Dwarf Frogs
4 Black Skirt Tetras
2 Apple Snails
We are over-filtering to roughly 40 gallons.
Suddenly, in the space of just a couple of days we had mass fish death. In one week we have now lost all
Rasboras and all but one
Danio. There have been no signs of disease and the water chemical levels have remained pretty steady throughout (ammonia appearing briefly after some of the deaths). The strangest part of it is that nothing else in the tank has shown any signs of stress or upset. The frogs, tetras and snails have all remained perfectly happy and healthy. There are no signs of bullying from anyone and there have never been any signs of injury.
Only one physical sign appeared on one
Danio and that was one small red patch on its side. I don't know if that was related in any way. The Rasboras showed very little sign of anything before death but the Danios (they held the longest) showed a lot of stress with clamped fins and drooped tails.
We took some water to PetSmart to have them test it in case our tests were incorrect but they agreed that ammonia, nitrates and nitrites were all normal. The only thing they could tell us that we didn't know was that the alkalinity was really low. They told us that this could cause a change in pH which obviously isn't good. Therefore pH has been our only lead... Wouldn't that be a killer of everything in the tank though?
One other thing was that we added a rock not long before the deaths began and weren't sure if that was somehow affecting pH. We have since removed the rock, but no changes have occurred.
The pH of the tank does seem to have changed since we first set it up. It started at about 6.8 and now sits at about 6.2. We haven't seen any change to this since we noticed that though.
Does anyone have any idea what might be happening in our tank? It's been horrible to lose this many fish as I never have before.
Any help is most appreciated. If you need any more information let me know.