With a 300 gallon tank you certainly have a few options. My thought process is as follows. A sump is simply a place to add more biological filtration and the main source of mechanical filtration and chemical filtration. It is also a great place to hide probes and skimmers and such but definitely not mandatory. If you are OK with equipment in the tank consider the following:
1. You can definitely use a canister filter and add carbon, filter floss and things like marine pure into the canister. You will need to use a very large filter or more than one and clean it out on a regular basis; i.e. every two weeks. Canister filters are no more nitrate factories than an unwashed filter sock, keep it clean and you will keep your nutrients down. You can then add in tank protein skimmers. Hydor makes a good one called the slim skim but be prepared to use more than one (i.e. like 6). Some people swear they can keep a reef tank without a protein skimmer; but with your set up you definitely should consider it.
2. With a 300 gallon tank you could consider adding a braced baffle and using it as a sump with some flow in and out, of the tank.
3. Finally you could consider a floor sump that is a display sump, like with a trigger system. You would not need to hide it, but as mentioned before, you may not have the floor space. What I am talking about is out in the open.
Bottom line is you can definitely do it but you will have some definite trade-offs. Decide what you can live with. A freshwater display with cichlids can be just as rewarding to some...
Hope this helps.
