Nitrate and Ammo

[Rikko]

I'm with Steve there. Test both your topup water and a small batch of fresh seawater and check for ammonia.. If they test clear, there's a problem. An established tank should never persistently have ammonia that high. I would suspect overfeeding is the culprit if everything else checks out.
I'm not sure about Mission's water, but if your water source is the culprit, an RO unit might be the way to go. A little pricey but ultimately worth it. Is there anyone in town that can test your tapwater TDS? If it's reasonably low you can get away with just a DI unit (Tap Water Filter goes for a little over 50 bucks) and the cartridge will last a good while.