Nitrate Problem

Hi, I hope someone can shed some light on this problem.

I have high nitrate (80 ppm) and phosphate (1.0 ppm) but zero ammonia and
nitrite, the ph is 7.8 and a salnity of around 30.

The tank is a 50 Gallon Jewel tank, it's been up and running for 5 years.
The tank has a couple of

Hermits
Emperor Tang
8 Turbo Snails
Star Blenny
Goby that never comes out from under its rock
Cleaner Shrimp.

I have prolific soft coral growth and an unknown alage or perhaps its a
seaweed, green and segmented, no doubt taking advantage of the nitrate
level. I also have plenty of purple coraline algae.

I pop in one algae wafer a day and some seaweed for the tang and every three
days half a cube of brine shrimp soaked in garlic.

Even with agressive water changes these levels remain high. I top up with RO
water that reads a tds of 50.

any help on how to manage this would really be appreciated.

thanks
Nath