I have a 60 gallon tank and was thinking about using eggcrate bought from Home Depot to make a reverse flow undergravel filter. The filter and it's riser tubes would all be sealed in place with aquarium glue so it couldn't be moved or dug under. I'll have two screened overflows plumbed through the back wall about 6" from the top to allow water to go to the external wet/dry filter. The filter is home-made, but is made from a 40 gallon tank and has 4 gallons of bio balls, two carbon filters and a protein skimmer. I have a 520gph pump and hope to add either a UV light or ozonizer later. There's also room left in the filter to add a refuge if I become convinced that's a good idea. I'll also have live rock in the tank with 1" of substrate on the filter. I was thinking with all the waste generated from the octopus this would help keep it out of the substrate and in the filter. Only the freshly cleaned water would be passing under the gravel, but I don't want to have any bad surprises later from this setup. Sorry this is so long winded, but I wanted to make sure I included everything. Thanks for any advice and help. Oh yeah, I'll probably be getting a Hawaiian Day Octopus once I find out what the rules are on having one as a pet instead of food.