Nitrate Question

Nitrates are the end-product of bacterial nitrification in our aquariums. They will continually rise in a cycled aquarium, and are removed w/ water changes. They're not so harmful in and of themselves, but do serve as an indicator of unwanted chemicals that one cannot detect w/ a hobby test kit.

Ammonia is acutely toxic to fish, and levels should read 0 with a hobby test kit. It is the waste product that is produced by your fish, converted first to nitrites (also acutely toxic) and then eventually nitrates.

The pH of your water should stay steady more than it should conform to any particular level (in most cases, breeding many fish is one exception). A fluctuating pH is harmful, but most fish can adapt to the ranges of pH common from household tapwater sources.

In summary, you should get the nitrate test kit. Ammonia and pH levels won't give you any indication of nitrates.

Yes there is direct relationship between pH level NH3 (ammonia) {toxic} and NH4 (ammonium) {less harmfull}.
As your pH level increases NH4 is converted to moire toxic NH3 due to the excess of H(hydrogen ions) in the water.
So it is important to monitor your NH3 level and not your NH4 level. Almost all your aquarium test kits measure NH3 and not NH4.
The rest is simply. Nitrates NO3 is the result of the nitrogen cycle, where NH3 is converted to NO2 by the beneficial bacterial. This toxic ntrite NO3 is then changes to less toxic nitrates NO3.
The NO3 must then be removed from the your aquarium (since your aquarium is a closed system) by regular water changes. Build up of excessive amount of NO3 can be fatal to your fishes.

Rohn

Most hobby test kits test total ammonia, both dissolved ammonia, NH3, and ammonium ion, NH4+, not just NH3.

Higher pH represents lower concentrations of hrdrogen ions, H+, in the water, not higher.

The nitrate concentration in the aquarium has no relationship to either ammonia concentration or pH, your LFS is incorrect.