If you mean 15000LPH flowing through a 100mm pipe then that is much too fast for biological filtration. Time inside filter isn't as important as velocity through the media.
For a large tank I would use a pair of plastic tubs/ponds. First as a settlement filter with a couple of bulkheads at upper water level flowing into second tub full of media, jap mat or sponge or whatever. Get a big tub and cheap media. Don't go nuts with baffles and chambers for sponge and different media etc, just leaves more chambers to vacuum the crud out of. Go as wide as you can find/fit/afford to slow the water down. If you need a chamber for carbon or whatever chuck a bucket in the second tub and T off return water to trickle into it.
Oh, and filter socks are handy to keep the big stuff out of the filter. Use the big ones and have spares so you can soak em in bleach to clean.
