"I can see there would be advantages of having two instead of one. When going with the two filter approach, is it as simple as adding the 2 separate capcities together to come up with the total you need? (For example: two 40
gal capacity filters for one 75
gal tank.) Or is it more complicated than that?"
Most manufacturers are very optimistic when they rate filter capacity. It is very hard to get enough filtration going by manufacturer specs. I have 110 gallons worth of filtration on a 26g tank according to manufacturer specs 8O A good rule of thumb is to try and get enough filtration to turnover the water in the tank 6 -> 10 times an hour. So for a 75
gph, try to get between 450 and 750
gph of water movement through the filters. You can accomplish this a lot of ways, so you have a lot of options. I personally prefer to have a cansiter filter and a
HOB filter for the reasons TankGirl brought up. Canisters are better for biological filtering and are more effecient at keeping the water healthy and polished, while
HOB filters usually suck up more water so they catch more waste floating around in the tank. The HOBs are also easy to get to, so if you need to add carbon or anything, you just drop it right in.
For a 75g, a good setup would be any combination of the following...
One Cansiter: Filstar XP2 or XP3, Fluval 304 or 404, Eheim ECCO 2235, Eheim Classic 2215 or 2217
One
HOB: Penguin 330, Emperor 400, Aquaclear 300 or 500
Any combination of those should keep you between $100 and $140 total for the filtration. Having two filters gives you a lot less to worry about.