I would buy a nitrate test kit (low level), and the nitrates should always register zero or below 5 ppm on the test kit. If they go above 10 ppm then you're feeding or fertilizing too much with nitrogen, and you should increase water changes and decrease feeding and eliminate nitrogen fertilization until they return below 5 ppm. Be sure you push the fertilizer tablets all the way into the bottom soil so they don't leak into the water.
A pH test kit is useless in Davis, it will always read high, but you will want one if you have soft water for when you start adding CO2. Keep the pH above 6.
An iron test kit is nice to check the iron levels (if you're paranoid and are adding iron to the water) but they're hard to find and often overpriced, and they won't tell you anything about iron levels in the substrate, which may be sufficient.
No other test kits are needed.
