I'm assuming by west 7 you mean west epoxy. If that is the cast here are a few things which may be of interest..
West epoxy is good stuff but expensive, and the standard hardeners can create a thing called amine blush which has to be sanded off or scrubbed with an ammonia cleaner - they do a 207 hardener that is more expensive but non blushing. If you get it mail order, expect nasty shipping charges most carriers call it hazmat and bill accordingly. Some people may paint the tank because epoxy in general doesn't like UV at all. There are hardeners that are better for this e.g. the West 207. If you want to get in depth with all of this try looking on the cedar strip canoe building websites (guess why my new tank has been delayed
). Epoxies also stick to wood much better than polyester resins hence their use in wooden boat building - nobody with any sense uses polyester based resins on cedar strip canoes or any other wooden hull for example..
One thing I will say is that epoxies have some irritant properties and some folks are allergic to them.
Polyester resin on the other hand is outright antisocial stuff - lots of styrene which stinks like you wouldn't believe and is really not good for you at all. DO NOT even think of doing it in the house! Don't do it on top of a chest freezer in the garage either, because the food will end up tasting of it even if the hermetically sealed door is never opened (Dad took a LOT of heat off Mom for that one) and I have no idea what any remanants of the styrene might do to your tank inhabitants.
You could paint over the standard polyester resin /glass with epoxy - in fact a good move since polyester resin isn't exactly impermeable to water. Epoxy is..
On balance, if I was building one I'd go for epoxy, the good news though is that there are cheaper suppliers than West, that may be worth researching. On my canoe I used Raka epoxy with the non blush hardener and saved big money over West...
I've not built plywood tanks before, but as far as I've read the strength is in the wood, not the epoxy, it only waterproofs it. If this is the case the 1/5 of an inch you mention sounds a bit like overkill though I'm willing to be corrected here..
Don't know if this helps or confuses you more, but I tried...
Rob.