Disappearing fish? help!?

I have a 75litre tank with 6 white cloud minnows, 3 zebra danios, 3 pearl danios and 2 chinese algae eaters (I know theyre not compatible but they were sold to me as 'peaceful, coldwater suckerfish' and the shop won't take them back). over the past couple of days I noticed one of the pearl danios was getting really fat but still active and healthy so it cant have been swimbladder. This is the fish that has disappeared. There are 2 caves in the tank, a small one and a very big one which the danios regularly swim in and out of. Where has my danio gone? It couldn't have been the chinese algae eaters because they are around 1.5" and 2". Please tell me what you think happened to my danio. Thanks in advance