Reasonably accurate test kits are either Salifert or Hach. RO/DI water is the way to go. An inexpensive RO/DI unit is AquaSafe which can be found on ebay. It's made in North Vancouver. You should test for phosphates. Also the amount and type of food you feed to your fish can put phosphates into the system. Using a phosban reactor and rowaphos works good for the phosphates. Nitrates are probably better reduced by frequent water changes.
Doug
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