Sandi wrote,
Mmm - there isn't anything that's EXACTLY the same in those but Hydra
vulgaris is the closest.
Tend to be of the mind that "if it ain't broke etc etc" but will leaving
them to sort themselves out harm the fish?

Hydra (freshwater anemones) can be killed by any copper treatment.
Aquarisol medication works well. Or, let an air supply run through a
copper pot scrub pad, or place it in the tank so a power filter return
flows through it. A medication called Fluke-Tabs (1 cap. per 20 gals.)
will also kill Hydra. Hydra's population growth is fast, and they grow
fast (up to an inch tall), at which size can harm or kill smaller fish.
Cause - your overfeeding and not doing enough weekly maintenance (20%
weekly water changes (at least) with a good gravel
vac)...................... Frank