Ummmmm..... not sure, could be tiny mites. Are you sure they are not little red worms
Anyway, in many cases, these little critters find their way into your tank through plants and other things you may introduce. You should look at your feeding regime, a lot of these free living animals thrive on waste food so if you are overfeeding, you could well be encouraging them to multiply. They shouldn't really cause any harm. If you really don't like them there, you could try taking all fish out and turning the tank up to about 35°C (if your heater goes this high) otherwise, the only way of getting rid of these things is to completely stripped the tank out and start again. Personally, I would just leave them in there and carry out extra tank maintenance and hoped they'd slowly disappear.
