Low level bacterial infection?
I have a Taiwan Bee tank that has been up and running with shrimp for almost a year now. Overall things are ok but once every several weeks or months 1-2 shrimp get taken out. I can't pin this down to anything, except maybe it was during a warm stretch and I ran the AC more often and the rest of the room was hotter. (The tank has a cooler, so the temps don't get above 70F) The symptoms are that their antennae become shortened or half-broken off, and they become lethargic. Sometimes they can stay in this state for a few weeks before finally keeling over. In the meantime, everybody else is doing just fine.
A while ago, I isolated one of the shrimp in a quarantine tank and treated it with Maracyn 2 (minocycline) for 4 days. It seemed to work, as afterwards that shrimp returned to activity. But unfortunately a few weeks later it became sick again, and another shrimp also got the same symptoms. (Antennae became shortened for a few days first, then became inactive)
So at this point I can either isolate both shrimp and treat them again, or just treat the whole tank.
I would rather not do the latter because everybody else is doing fine and I read conflicting reports on whether Maracyn 2 will nuke the filter bacteria.
Is there anything else I can add to the tank to be more anti-bacterial or preventative?
I already have low pH (6.0), low temps (68F), some Indian Almond leaves (not that much, tho), and I change 20% the water every 1-2 weeks.
Other things I can think of doing are: Dr. Tim's Eco Balance, Melafix, regular H2O2 treatments, some different kinds of food, etc?
Tank parameters: pH = 6, NH3 , NO2, NO2 = 0, GH=5, KH=0, temp=67-69F