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I have a 2ft, 20 gal tall planted tank. It contains a well aged piece of bogwood and various crypts, swords and vallis. I currently have 2 blue rams, 4 emperor tetras, 5 rummy nose tetras and 4 corydoras trilineatus. I use Excel 3 times per week and Aquasonic Daily Gro every day. There are two 20w flouros on for 9 hrs per day, algae is minimal if any. Water temp is 26C, ammonia 0, nitrite 0, nitrate 15/20, pH 7, don't know GH/KH but isn't extreme in either direction, or so I'm told.
I do a weekly 35% water change and vacuum the 2/3mm gravel on a regular basis. I have an Eheim 2010 internal filter that is regularly maintained. The tank has been running well for 6 months now (no new fish added since then), everyone shows good colour, feeds well, the rams spawn every 4 weeks (without joy). Never had any problems with the tank, used Melafix for some slight fin damage on the female ram a few months ago, no other medication. I read all the magazines and books I can on fishkeeping.

The tetras are fed micro wafers, the corys get Hikari sinking and algae wafers on rotation with Tetramin tropical tablets. They share one tablet broken into bits between them when the lights go off, once a week they go without. I check with a torch to make sure they feed properly and they do.

Here's the problem. In the space of the last 24hrs one of the corys developed a redened, slightly swollen belly and promptly died. The red colour was coming from inside the fish, not the skin. It showed no other symptoms. I removed it from the tank and did a precautionary 30% water change (I used Prime as always). The others seem to be ok.
Any ideas as to what caused it? Thanks.

This is not an easy one to diagnose, sometimes small pockets of poisonous gasses are formed by rotting uneaten food and if ingested by fish can be fatal. Why I am erring in this direction is the fact that you feed algae wafers, although they may appear to be eating them I think is more likely that they are just sifting through the particles as they dissolve. Algae is not really part of a Corys staple diet. They may naturally consume some during the course of foraging and looking for food in the biofilm that forms on submerged roots, rocks and plants. Unless you have dedicated algae eating fish species I would stop feeding the algae wafers and check the substrate for gas pockets by probing around the tank with a piece of cane or some other probe.

Ian

I take the point about the algae wafers, I'll give those a miss from now on. I have checked for gas pockets with a fork and haven't found any.
Maybe it's just one of those things. Thanks for your thoughts.