Hydra get introduced into aquariums with plants and snails. The hydra attach themselves to plant leaves or snail shells and when the plants or snails get moved into a new tank, the hydra go with them. Hydra are not a problem to adult fish but will eat baby fish. In most aquariums, Hydra struggle to survive due to lack of food, but they can last for months without food and they simply get smaller. When food becomes plentiful the hydra grow rapidly and spread.
If you are really concerned about them and there are hundreds of them, you can use Copper sulphate will kill Hydra.
-----------------------
Water Sprite (Ceratopteris thalictroides/ cornuta) is a good plant to try. It grows rapidly on the surface but can also be planted in the substrate.
-----------------------
What do you feed the fish?
If the fish has an underlying health issue (weakened immune system or something else caused by inbreeding), it could just be run down. It certainly doesn't look that healthy, and this could be caused by malnutrition, which can be caused by diseases or malabsorption issues (the fish can't digest food well).
Raw/ cooked prawn can be kept in the freezer. You take one out and defrost it, remove the head, shell and gut (thin black tube in body) and discard these pieces. Use a pr of scissors to cut the prawn tail into small pieces and offer 1 or 2 bits at a time. Feed until the fish is no longer eating and remove any uneaten food. Do this once a day and offer other types of food throughout the day.
Feeding the fish lots of high protein food can help them build up reserves and might help it recover.
You can add a vitamin supplement to food. Try to use a fish vitamin supplement. If you can't get a fish one, look for a bird or reptile vitamin supplement. The fish vitamins can be added to food once every day. Bird or reptile vitamins can be added to food once a day for 6 days but do not use it on the 7th day.
You can try adding juice from orange and purple food (orange & purple carrots are good sources) to the food. The orange foods have betacarotene in, while the purple foods have a couple of other anti-oxidants in. These are good for boosting the immune system and might help.
-----------------------
You can try treating the fish for tapeworm, gill flukes & thread/ round worms. Praziquantel (available from most pet shops) will treat tapeworm and gill flukes. Levamisole will treat thread/ round worms.
You treat the fish once a week for 3-4 weeks and do a 75% water change and gravel clean the substrate 24-48 hours after treatment.
-----------------------
You can try adding carbon to the filter. If there is a chemical in your tap water it could be causing problems to the fish and shrimp. Carbon will help remove chemicals from the water.
You don't want to use salt (sodium chloride) for too long in tanks with Labyrinth fishes because it can damage their kidneys. And Bettafix can case issues in tanks without filtration or aeration because it leaves an oily film on the surface.