I've got a 10 gallon tank. I don't have tests for nitrates and nitrites, but I'm using API ammonia tests and they're coming out 0 (I'm going to invest in the other tests very soon). It's a freshwater setup that has been running for about 2 months. There are currently 2 Julii corys and 3 Otocinclus in it. The water is currently about 77 degrees F. There is one Java fern in the tank that appears to be thriving. I'm using a Whisper filter rated for a 10g tank. The tank does not receive natural sunlight, the artificial lights are on for about 8-9 hrs a day. I use a vacuum to clean the tank once weekly, usually 2-3 gallons at a time. I changed out one gallon yesterday and had changed two gallons just a few days prior. The corys get a sinking pellet every other day, and the otos get a slice of blanched zucchini every other day. I have not noticed any abnormal behavior or physical abnormalities in my fish, which makes this all the more upsetting. Beyond changing out a gallon last night, I have not done any other treatment.
I changed out two gallons of my tank water and left the fish for the weekend after feeding them. When I got back, one of the corys was dead with absolutely no sign of what had happened. There was no sign of sickness prior to my leaving. All of the other fish seem pretty normal so far. My question is, should I be doing anything special to try to prevent another death? How long should I wait before I add any other fish? I know that the corys do better with more buddies, so eventually I would like to get them two more fellows. One thing that I have noticed is that there is some sort of white stuff developing on the sides of my tank. I scrub it off with an algae sponge when I see it, but there seems to be more of it since I left this weekend.