the above looks far more than adequate IMO. i have had container ponds from 20-150G, and never had any filtration, just circulation, and plants. what do you mean by "nutrient overload", did your water turn green? that's just too much suspended nurients that cause the algae bloom, but if you just stick it out for a week or two, the algae will use up all the nutrients and die off(this won't happen if there is a continuing source of nurient rich water, like possibly your tap). after the algae bloom dies off, and begins to decay you will have very low oxygen(it's used up in the decomposition process of the algae i believe), but shotly thereafter you can add some fish. i perform water changes very rarely( maybe a couple times a year to remove the several inches of sludge in the bottom). the only equipment i ever use is a pump, most of my small ponds have been run with old PH's. also , i have tried the thing with putting the gravel on top, but what i do now is just plant in straight gravel( i use a pretty fine grade, like special grade reef sand)and just let the plants get their nutrients from the water, which will in turn prevent algae blooms. kinda like mangroves in a reef tank, just use the plants to remove the nutrients that are causing the problems.