Filtration problems and bacterial blooms
Hello, I need some help, I am new to the forum so if I post in the wrong place I apologize, still trying to figure things out.
I have had a recurring problem in my tank regarding bacterial blooms. I have two 45L tanks, one has been running smoothly for about 7 months now with no problems, but the other one is giving me some problems. I bought the tank, set everything up and then let it cycle for about 3 weeks, I then tested the water and all the parameters seemed fine so I added some silvertip tetras and 2 Corydoras. Everything went fine for about a month after which I woke up to an extremely cloudy tank and a bunch of dead tetras, the Corys were fine so I took them out, cleaned everything (as I thought it might be something worse than a bacterial bloom - new to the hobby) then I set everything back up with the two corys in the tank and left that for about another 3 weeks to ensure that my filter gets colonized and everything runs smoothly as I didnt want another catastrophic event.
After the three weeks the parameters was fine again so I added some black phantom tetras with the two corys and again everything ran smoothly for about another month. Woke up one morning to a bunch of dead tetras and two survivors - the corys.
I then took some of my water to my nearest fish shop and asked them to test it for fear of it being my tests but they said everything is good and said it might be my filter.
I have a internal DoPhin F800 filter running in this 45L tank with an airstone and heater.
The strangest thing is that the other 45L is running on the exact same filter, heater, airstone and pump and I have never had this problem in that tank. Its been running for much longer than my problem tank and I know its definitely not over feeding that causes the blooms as I feed both of the tanks and I feed the problem tank a lot less on top of this.
My question is: is this my filter that might have something wrong with it like the fish store suggested? They advised me to add some seachem matrix to the filter in the sponge compartment and this is what it is currently running on, again with the 2 cory survivors.
Is it possible that there might not be enough surface area for the bacteria to grow on? Will adding a small sponge filter along with the internal filter help?
My other thoughts was that it might be the corys. Iíve read that they can release toxins if they are under extreme stress. Is it maybe possible that the tetras might have stressed them out enough for them to release the toxins?