i was just at petco and they are having a one dollar per gallon sale with there tanks i think only up to 55 gallon but i was just wondering would a 36 18 16 (l w h) be a good size to make a sump/ refugium? how much water do i need to have like how high does the water level have to be?
you can have it as high or as low as you want. If the baffles are set too high you will get an over flow. Thats why in my drawing my baffles dont go to the top. You need to figure out how much water will drain out of your tank when the return pump is off that will tell you how high the baffles can be. That size tank sounds like a 40 breeder that would be fine a 50b would be better 36x18x18.
I have a 50 gallon breeder tank as my sump/refugium and it fits nicely in the stand I built for my 120 gallon tank (4X2X2). It leaves a little room leftover for the light, fan, height of the skimmer, etc. What are the dimensions of the 55 gallon tank in your garage? That would probably work for you instead of having to buy a used 50 gallon tank.
Here are a few pics of mine. The tank is a 60" 100 gallon and the sump is a 30 gallon long. All joints are glued with exterior glue and screwed with drilled and countersunk galvanized deck screws. The stand is primed with Kilz then painted with white enamel and is waterproof. The exterior is 1/4" oak plywood with solid oak trim and unfinished doors ordered from Home Depot to match the rest of my house. Finish is Minwax Golden Oak with 5 coats of low gloss spar varnish urethane over it.