Telling planaria and hydra apart
After dark, run a torch beam across the glass, when both creatures come out to hunt. Planaria are flatworms a few millimetres to over a centimetre long that glide in one smooth motion, keeping a steady outline; look for the rounded, spade- or arrow-shaped head and, on pale species, two dark eyespots. Hydra look like tiny pale flowers stuck to glass, plants, or wood: a thin stalk topped with a ring of fine tentacles that drift with the current, and the whole animal contracts into a blob when touched or when the glass is tapped.
Where they come from
Neither appears from nowhere. Planaria usually arrive on new plants, wood, or livestock and multiply fast wherever uneaten food collects — a growing planaria population is often a feeding problem wearing a disguise. Hydra hitchhike the same way and bloom when the water is rich in the tiny live foods they sting and eat. A tank can carry a handful of either for months without the keeper ever noticing, because both stay small and shy while food is scarce.
Do they matter for your shrimp?
Hydra can sting and eat newly hatched shrimplets, and a large hydra bloom will thin out a batch of babies, but adult shrimp simply walk around them. Planaria are the more serious tenant: they actively hunt, sliding into the soft bodies of freshly moulted shrimp and taking eggs and weak animals, and a trap left overnight in an infested tank often comes up crowded. For a display tank with adult shrimp, both are mostly cosmetic; for a breeding colony you are raising to sell or expand, planaria are worth removing properly.
Reducing them without nuking the tank
Start with housekeeping: vacuum the substrate where food settles, feed in a dish you can lift out, and cut rations until leftovers disappear within the hour. A glass planaria trap baited with a small piece of protein food, set at lights-out and emptied each morning, pulls numbers down steadily. Any chemical route needs care in a shrimp tank — check that a product is sold as shrimp-safe, and never assume a wormer meant for fish-only tanks is safe around invertebrates. Fish that hunt flatworms will usually hunt baby shrimp with the same enthusiasm, so they are rarely the answer in a breeding setup.
When to get a second opinion
If the creatures you are seeing do not match either description — segmented bodies, bristles, or anything attached to a shrimp's shell or head — the identification matters more than the treatment, because the right response differs completely. Photograph the animal against the glass with a coin for scale, note where it sits and how it moves, and get the ID confirmed before adding anything to the water. For that unfamiliar find, check head shape, movement and tentacles first, then put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.