Help me I've got fleas
I have a 6 gallon fish tank with 4 fancy tailed guppies, a Chinese algae eater, and an ugly sucker fish of some sort. I do not know the name of the sucker fish, but believe that after I bought him and put him in my tank, is when the problem started. I have some sort of fast breeding crustaceans taking over my tank. I have broken my tank down and cleaned and cleaned and cleaned it on several occasions, but they keep coming back. They seem to start down in the gravel, then you can see them look like they are floating up to the top, just to swim back down. After a week or so, they will start up moving and down the walls of the tank, almost marching like an army.  I have asked several pet stores and they sell me stuff to treat the fish with, even though I have tried to tell them that the fish are fine. The fish will not eat these things and these things do not attach themselves to the fish. I even dipped some out of these things out of the tank and took it to several places that sell fish. They don't have a clue what they are! I looked at 3 under a 5X's magnifying glass. They are light tan color, shaped like a football with black dots (I could not see any legs), and feel like a piece of sand. I hope you can help me with getting rid of these, because I really do enjoy having a fish tank. 
< I think you have a species of daphnia or water flea. Some small red species are edible to fish, but others are hard and fish do not like them. I suspect that your gravel or some live plants may be responsible. If you used a inexpensive natural sand then I think the local river bed may have contained some of these critters and it took them a little while to reproduce in the numbers you now see. The red ones feed on algae particles in the water. In fact some green water would be required to keep them alive. I have seen something similar to what you describe in with pond plants and duckweed all the time but I am not sure what they eat. Since you have such a small tank I would take the tank apart. Then I would wash the sand thoroughly in a five gallon bucket along with the decorations. Carefully add a cup of bleach and let everything soak. The water fleas should be floating to the top of the bucket dead. If not then add another 1/2 cup of bleach. If everything is dead then I would get some rubber gloves and wipe down the interior of the tank with the bleach mixture. Rinse everything good at least three times. Put everything back and check the chlorine levels in the water. Add a water conditioner to remove any remaining chlorine residue. Your tank has now been sterilized and you have no biological filtration so you will have to carefully watch the ammonia levels until your tank gets cycled again Don't add any of the water from the container with the fish. Pour the fish into a net and place them back in the tank.-Chuck>