I've been told that our water supply is high in phosphorus making tanks
algae prone. Is there anything other than RO changes that can reduce the
phosphorus?

I was in this situation before... are you high on N and P or just on PO4?
Anyhow, the whole trick was to run the tank below tap conditions... lets say
you have 2ppm of PO4 in the tap, you can pound the tank with CO2, K, NO3 and
trace to knock PO4 levels down in the tank... if you change 25% of water
weekly, you would be adding 0.5ppm so as long as you can get the tank to
suck up more than 0.5ppm per week plus whatever the fish food adds you
should see levels drop over time... you sorta get to turn the whole thing
upside down and look at water changes as fertilizing the tank... it
disgusted me for a while to think my tank water was cleaner than what I was
drinking. If you're high on N and P like I was, 30ppm NO3 and 2ppm PO4
expect some algae for the few weeks it takes to get a handle on things...
the way to cope with "rich" water is to change a little less weekly and keep
fish loads really light... I wouldn't go buy an RO system or anything like
that its doable, just takes some practice.

Cheers,
Jeff Ludwig