How bad is my fish tank for my fish?

i have 3 pictus catfish 1 oscar and an algea eater all in a ten gallon tank.. i know its really small for all of them and they are all still small.. well Oscar is about 5 in ches.. how bad exactly is it that they are all in there.. because theyre all in the small tank i do water changes twice a week and that seems to keep the level somewhat balanced.. the nitrate levels are still really high.. i use easy balance by tetra in the tank but it doesnt seem to help that much. i need help.. help!