With pretty reasonably/regular rain this would work- 500G is a lot of water and it sounds like it's catching from the roof. How much does it rain in Guam? Seasonal?
Tarpon (well, at least Caribbean version) are obligate air-breathers, so no matter what the O2 levels in the water they're groovy. Tarpon are also incredibly tough- quite comfortable in some pretty vile and/or toasty water. Their scales are heavy and their skin slime is thick, viscous.... and stinks- you can't wash it off your hands without serious effort. Physically and chemically armoured against half-assed husbandry