I use Phosban. Until recently this was just in a filter bag hung in the
sump but I've just bought a reactor: I'm hoping I'll get more milage
out of the Phosban and the reactor will pay for itself in the long run.
It only needed changing every 3 months or so before but it's expenisve
stuff.

I've also just started running some carbon in a bag in the sump just to
see what happens. Can't honestly say I've noticed any difference yet.