i had one of those (ok, i bought it for my kidlett and it kinda became mine, lol) and the fish get huge in there! like, pressing against the sides of the tank huge. so yeah, overstocked is right, lol. only problem is that the stuff the fish are made of seperates as it expands and the colours don't stay the same. helpful hint: use cold water.
Just as a word of precaution my sister bought one of these for her son and her cat got hold of one of the expanded fish and chewed it. Needless to say the kittie is no longer with them and the vet said it's untimely death was caused because of the ingestion of the material they are made of.