There are phosphate tests for fresh water aquariums. API makes them, and other companies. Look on line, if the local stores do not carry them. Drs Foster and Smith, Big Als and others.
KNO3 is the major source of nitrates for people who dry dose their tanks. It is usually used as an agricultural fertilizer, so you might find a bit that way, and it is also sold as a stump remover. Make sure you read the label, not all stump remover is KNO3. Make sure it says potassium nitrate.
Fish are not causing the nitrogen problem. Fish food is.
Feed only 1/4 the regular amount for a couple of weeks, and skip one day of feeding entirely. If the fish will accept vegetables, then feed them salad or lightly cooked vegetables at least once a week. High protein foods contribute the most nitrogen to the tank.
Do not go by what the testing label or instructions say about what is a safe level of nitrate. I have found my fish to not be too happy when the nitrate level from fish food and other decomposing things gets up there, but they do not seem to react the same when I dose KNO3 as a nitrogen source for the plants. Also, different species of fish are sensitive to different levels of nitrate. I generally keep all the tanks under 20 ppm, and aim for 5-10 ppm either from fish food or from fertilizer dosing.
If you do smaller water changes, frequently, or one large water change less frequently does not matter.
If you are not removing nitrogen as fast as you are adding it then the levels will rise.