You may want to buy three, like they said you could have one on the 20 and 2 on the 90 and you'll be all set, at that price you can sell them used and not lose a cent (that always helps me to justify buying something). it will be your cheapest way to go and give you enough filtration for your needs. I have an AC70 with the sponge, scrubbies and some filter floss and it works great for me, i have it teamed with a magnum 350 but its on a 150 gallon, so two of them on a 90 would work fine.