NONE!

I think testing is a real waste of time unless you have major problems.

I have very healthy ponds and fish and never test the water.

Of course, it gives Newbees something else to worry about. LOL

I read on another forum...that you take your number of drops and multiply by 17.9ppm for both Kh and Gh.
The tetra test doesnt say to do that...but that puts me in the right numbers...so thats probobly right.

I don't do any testing either unless there is some issue to deal with. GH is "general hardeness," a measure of calcium and magnesium in the water. KH is "carbonate hardness," a measure of bicarbonates and carbonates in the water. Carbonate Hardness is a component of alkalinity, which is how well the water can resist pH changes.

Both GH and KH can be expressed in German Degrees (dH) or mg/l (hint: mg/l and ppm are the same thing). 1 dH = 17.9mg/l. That's where your 17.9 comes in. If your test gives you results in dH and you want to convert it to mg/l or ppm, then you multiply your result by 17.9. Just follow the directions on the Tetra test kit and convert it if need be. I don't believe the Tetra test needs any drop-dependent-multiplication, but I haven't used it in a few years. Just follow the directions and you'll be fine.

FYI - a "typical" GH is around 5dH or 100ppm. I wouldn't worry too much about the GH though. GH shouldn't need to be adjusted unless there is a problem. KH, pH and CO2 are all linked. Unless you are having a problem with stabilizing your pH, I wouldn't worry about your KH either. Expert water gardeners that are trying to grow difficult plants are the only ones that should be overanalyzing their KH values.

I bought my in-expensive test kit at on-line . I think it was september.
Im pretty sure all the reagents would still be good. But how would you know?
Do they have a shelf life?
Mike