You cannot compare the growth of aquatic plants in a fish tank with that of orchids. Most aquarium plants require basic water, certainly no less than neutral. Most orchids, certainly Miltoniopsis, require slightly acidic conditions. While calcium is essential, a slightly acidic pH is needed...say, around 6.5. Many paph's require basic conditions, but I don't think your filter compartment would work. Yes, the water is relatively oxygenated, but not enough for the roots. Some people have had success growing paphs in semi-hydro culture, but I am not one of them. You could try the tank water in semi-hydro, but the bacteria would probably kill orchid roots.