I set up my tank 4 months ago using Nutrafin Cycle, and added fish 1 week later So far none of the fish that I originally bought has died One of my corys has always spent more time in midwater, and recently started to hang off the surface of the water vertically while flushing air bubbles through his gills (at this point I tried doing frequent water changes and feeding peas, not sure if that did anything) For the past week he had trouble sinking to the bottom and staying upright. I found him floating on his side or belly a couple of times but he always swam away normally when I knocked the tank. He is also much smaller and whiter than the other 2 corys I bought at the same time (they looked the same when I bought them). In the last two days, he spent most of his time at the bottom of the tank not moving, and would only wiggle slightly when other fish nudged at him. He probably only has a few days left since he doesn't eat anymore. None of my fish look sick (no flashing, spots, streaks, missing fin/barbs, etc) I need some ideas on what happened to this cory, and how to prevent it Thanks! Tank details: 10 Gallon, Aquaclear 20 with sponge, purigen, and biomax insert 7.6 pH, 0 Ammonia, 0 Nitrite, 15 Nitrate (freshwater master test kit), 75 degrees The last time I tested a spike was around 2 months ago, but no fish died (not even the baby platys) 3 adult Platys (and a few babies) 3 Albino Corys 6 glowlight rasboras The tank is planted with hornwort, water wisteria, and anubias (and some algae and snails that hitched a ride when I bought the plants) Edit: Update The sick Cory died after I got to home from school today. I noticed that the bottom of his body caved in and made an oval dent under where all his organs are clustered (behind the head before the body tapers off). Is this normal or a sign that parasites have escaped the corpse? If so what type of parasite might this be and what meds should be used? I know I should isolate sick fish, but I don't have a separate hospital tank and don't want to kill him faster by moving him into a bucket. My other fish didn't bully this cory, the platys just poked around him occasionally because they think the cory found food.