Need some help re nitrates


Reefing newb
I’ve had my 30 gallon tank for over 3 years now. Although my fish seem to be doing fine, I’ve always had a problem keeping the nitrates down. I’ve recently (Dec 2014) changed the bottom from loose crushed coral to sand and just replaced by dual T5 HO fluorescent lighting to a brighter Coralife Aqualight LED. I’m not sure if I have been missing something fundamental as I can’t seem to get the nitrates down despite frequent water changes. I think it has been responsible for killing off a long-tentacle anemone (which I’ve had for about 6 months) and my frequent failures of keeping zoanthid coral alive for longer than a few weeks. I’ll give you the basics and maybe there is something I have been missing or doing wrong all along.
30 gallon tank – water changes approx. 1-2 times a month often 7 gallons at a time- sometimes 10 gallons.
Current parameters: 0ppm ammonia, 0ppm nitrite, pH 8.2, ~40-60ppm nitrates
Livestock: 1 clownfish, 1 six-line wrasse, 1 royal gramma, 1 purple fire fish, 1 leatherback urchin, 1 long-spine urchin and a few hermits, snails for CUC. Also have a starfish and 3 heads of mushroom coral.
I feed them 2x daily with a small pinch of dry flake –omega one with garlic.
Like I said, fish seem healthy, I’ve had 2 clownfish since I started the tank- 1 has since disappeared and presumed dead L. I would just like to clear up the nitrates and I think I’ll have better luck populating the tank with various coral- which is what the intent was from the beginning.
Also, if you have any ideas for good starter coral, I’m all ears.