I agree - I'm not a big fan of undergravel filters. There are many options in terms of bio filtering that are far more effecient (as mentioned by Xenos Wet/Dry) also Fluidized bed filters (widely used in the aquaculture industry), LR
, various excellent additives for Canisters (like Ehifsubstrat). So my intial reaction is 'bag the undergravel.'
As for turnover - for a 225, depending what you are keeping I would aim at a minimum of 10 times - you should be considering a Blueline HD100, an iwaki 100RLT or perhaps an Ampmaster.
The T3 is an excellent pump and is pressure rated but if you are pumping it through a few tees and elbows and pipe to tubing connections or canister-type filters or UV
's it is likely to be pumping more close to the 400gph range which IMO
is way to low for a 225.