Ok so at the nature park we have started building our exhibits and one of the items that we are going to have is a saltwater touch tank that folks can approach and it will have some critters in there ie flounder, starfish, horseshoe crab banded blue crab etc. the tank holds about 50 gallons filled. it was buit before i got here and currently has an inline charcoal filter and a coralife external pump on it it is on a rolling cart and i know i want to add additional volume underneath a protien skimmer etc but i am needing advice on what the best method is for filtration both biological and mechanical. keep in mind that there will be no live rock up top so all filtration means must take place underneath. i will try to post pictures in a bit