Phosphates can be injected to the system at many points....alot of those points were mention such as source water. Another area for phosphates to be introduced is through your food source which probably alot of us including myself overlook.....Frozen food is a big culprit in the hobby......
Water changes will help, along with increased water movement to suspend junk in the water column so it can be efficiently removed, probably the cheapest way is through exportation with macros, and you can supplement that with running a Phosban reactor......I run macros and supplement with the Phosban reactor, but choose to run RowaPhos in the reactor.....Alittle more expensive, and can have side affects if to much is used as well......