I have a 300lt tank with 5 bullies in it. I have no substrate so that it is easy to keep clean. Just a couple of pieces of wood for shelter. The fish are fine. However two alien creatures have turned up, both small, one a worm type creature that wiggles hard out in figure of eights, perhaps 4-5mm long. This one is not very common. The other creature, however is very common, after a wipe down of the inside glass it gathers on the bottom, perhaps in their hundreds, it is very small, about 1-2 mm long and moves in a jerky manner. Does anyone know what these creatures are?

Sounds like Bloodworms and Daphnia.

Where do you collect your water from?

Bloodworms are distictivly red

Daphnia are small, about the size of a flea, infact they are known as water fleas. They swim with a jerky motion

vairious specises of small aquatic worms hitchike into tanks on wood and weed collected from outside.I also belive aquatic animals appear from thin air.I once ad a one gallon jar outside with some nutrince to grow some green water to feed my fry and i ended up breeding rotifers of some kind.I dont know where they came from.I have seen the small worms you are talking about in some tanks before.I dont belive they pose any threat except maybe to small fry.