i think we ALL have been owned, and it is time to tone this one down a bit!!!
first of all, everyone seems to want to be arguing with one or 2 people because they THINK he/she is wrong and they are right. WRONG. doing so just for the sake of instigation is neither helpful nor courteous
this will not be tolerated. i might believe a good excuse, but IMEO he has made more sense and been more correct than those attacking him.
without insulting and getting into details and specifics on species i can see this much. 24 inches is merely 2 ft, not 3!
60 inches is 5 ft, and 60 some inches is only a little more than 5 ft, so why are we arguing here? it is not like the tub would be 9 ft. THEY STILL NEED THE SPACE, and more than 5 ft of it
now, onto the fish, a 300 is barely what i would begin to try and keep one single epaulette and no more!! NONE! but this one is merely my opinion. However, if you are talking multple sharks with rays, a 300 ins not even worth calling a possiblity. again, my opinion but it is ithe same one that any EXPERIENCED or at least reasonably educated hobbyist would agree with. and being built like this, 300 gallon is nothing. get a good 8 ft or more swimming area. more, for active swimmers and 8 ft for the more inactive species. IF you want to do a pond, get a liner and dig your own! they are expensive, if even available, in the size you should be considering. so making your own would be highly advisable.
filtration, IMHO, a good, oversized skimmer and sump sytem, with a good large wet dry would not be a bad idea. depending upon the species and or the rays, live rock will vary greatly. however, several good strong powerheads should still be used.
hth