[Sk8r]

Always have enough salt and enough ro/di water to do a change, have AMquel handy, and in general come here to ask---you almost always can get a good answer fast if you put the problem in your subject line: the people who know that problem will spot your post and come to your rescue usually within an hour, sometimes within 5 minutes.

Re food: shake the food into your hand before putting it in the tank: that way the mistakes get caught before they hit the water.

If something pollutes your tank, charcoal for tanks in a bag, placed in the flow, will remove ammonia and many other pollutants.

A 10 to 20% water change will lower nitrate.

A strip of polyfilter placed in your water flow will 'diagnose' a pollutant like organics, [shudder] metals, etc, and a larger amount of this expensive stuff will actually remove it.

These are your several basic quick fixes.

I use a shot glass full of tank water to dissolve frozen food...which puts less mess into your tank if you choose the right frozen for your fish.
Crook your finger over top, discard most water, then put dissolved frozens in tank for feeding, rinse glass, set aside.

HTH.