I have a very small tank (15 litre), which until recently I had 6 guppies in. They were all very happy and then suddenly we lost 5 of them over the course of a week - it's a bit of a mystery as I test the water weekly, do 10% change weekly, only feed them a tiny amount every other day and there were no signs of illness on the fish (other than their behaviour - hiding, hanging around at the bottom of the tank). The nitrates had gone up a little, but not dangerously. Anyway - I have one surviving fish - he seems to be doing quite well and has been alive for 2 weeks now, since the last fish died. Not sure what to do now - should I leave him by himself for a while longer, or get him a couple of playmates? Is it ok for him to be by himself? I know guppies are community fish, but I don't want to add more if my tank/water is dodgy!