you can still put decor in the tank with a bare bottom, but obviously you will need stuff that doesnt need to be buried in the gravel or need the gravel as padding (big, heavy, or sharp rocks). dont worry about flow rate. i know a lot of people will give minimums, but it doesnt matter how many times a filter turns over the tank in an hour, its what the filter does with the water while it cleans it. if a filter is just pushing water through you cant get much done to the water, but something like a fluval that has a relatively low gph rating can handle a bigger tank because it is doing more with the water while it has it. my guide is to take whatever a filter claims it can handle and cut that in half, that is about how many gallons of a moderately stocked tank that filter can handle. you also need to rememer that youre not really filtering 55 gallons of water, you will be cleaning up after the fish. so things like bioload and feeding habits are also very important. tank size is just one factor when determining the filtration needed. also remember that no matter how much filtration you have, if you dont do proper water changes, you are still causing low water quality. my goldfish tank gets a 50% water change once a week. they are growing like crazy. nitrates and pheromones limit growth. frequent large water changes keep both at low levels allowing faster growth. biological filtration simply takes ammonia and turns it into nitrites, then into nitrates, it is then up to you to remove the nitrates, which needs to be done with water changes. fluvals are great for mechanical and chemical. in the bottom tray or two of a fluval i cram in as much filter floss as i can, it functions as a great fine mechanical media. biowheels are the best biological filtration out there. with 30,000 times as much oxygen in the air than in water, they are supreme to any submerged biological media, and any submerged bio media can become clogged and then start to actually choke out the good bacteria. the fluvals also allow for a lot of carbon for chemical filtration.