Answers to all your questions:
"Can I buy fittings at Lowe's that will screw into these holes???" If you can find 1.5" thread-barb fittings, Lowes or Home Depot is as a good source as any...or a local plumbing supply. If not, you can get 'em from any marine store. " Do the fittings have to be polyethylene or will other types of plastic such as PVC be OK???" They don't have to be PE (I don't know whether PE fittings even exist), but I'd go with nylon or marelon instead of PVC. Reason: for some reason I've never been able to learn, nominal 1.5" thread-barb PVC fittings are actual 1.5" thread x 1 5/8" barb...which makes 'em d'd near impossible to get a hose onto. Nylon fittings, otoh, are 1.5" barbed. Warm the hose with a blow dryer (don't use a heat gun unless you really know what you're doing with one, 'cuz you only want to warm the hose, not heat it), smear a little dish soap--or better yet, K-Y jelly--on the barb and the inside of the hose. Double clamp the connection, putting screws on opposite sides of the hose. " Should I use tools to tighten or just hand tighten to avoid cracking the tank??? Just one full turn past hand tight, plus only enough more to aim an elbow in the direction it needs to go...no more, even if threads are still showing. " Teflon tape??" Yes. "Finally the tank is 3/8" thick and seems very strong do I need to build a plywood box to support the tank??? No...just make sure it's well supported on the bottom. " Right now I am planning to build a 1/2" plywood shelf to sit the tank on then strap it in place with nylon straps. Sounds like a plan to me.
