FIG·5
FIG·6
FIG·7
HYDRA—a polyp, fastens itself to plants or glass. It has a variety of shapes and is hard to distinguish particularly on plants. It attacks and quickly drugs fish by injecting a poison. Hydra can be killed by removing all fish and scavengers and raising temperature of tank to 115° F.
AERATION
Replacing consumed oxygen by blowing a stream of very fine air bubbles through the aquarium water is frequently necessary in aquariums, where plants receive insufficient light or are not thriving, where the air surface is small (depth greater than width), where crowding is necessary, etc. Even in healthy aquariums, where plants are thriving, conditions will be improved by an hour or two of aeration daily. Many types of aerators are available.