WooooWha...............
For starters just about every large aquarium across the country uses Sand filters ....... with that being said most rays enjoy a dsb so all this talk of marine sand being bad is just plain garbage. However they are very much correct about your tank size, your filtration size and type................. your biggest problem tho is complete lack of marine animal care.
I've seen 1st hand Dogfish living well in a 300gal tank w/ a 125 gal sump containing 100+ lbs of LR and a 4" deep sand bed - no skimmer - and he uses de-chlorinated tap water. He also has no lighting at all. Not all sharks grow to 14'.
Phosphate eating bacteria really??? Even if it exists it has no effects on an F/O tank - heck who has a po4 reactor on an fw tank? Who uses r/o di water in FW? Does anyone have a protein skimmer on their FW? My 12-24" FW's live in de- chlorinated tap water and my 16" Caribe is almost 10 years old. PO4 in a fish only tank is a non issue SW or FW. The issues with F/O are the same as FW - Nitrates 20ppm or less via large volume w/c's which also becomes a money issue with salinity. If you can afford to buy lottsa salt no worries.
Go back to the drawing board - rethink filtration and tank size - research research research - what you want to do is not impossible.
you're like the only one that on here that says I don't need a skimmer lol , and I think i really don't need one because a lot of people I know say you don't need one, and they don't even have one on their tanks. With their tanks thriving, I don't think a skimmer is a necessity.
Maybe i'll just get crushed coral and remove my sand. And that's it. I have a buddy doing that. and it's working great for his sharks and eels