i also dun know, but as i read, i think ammonia and nitrate test strips are more important ?
Don't use test strips if you can.
Titrated test solutions more accurate.
You needs tests for
Ammonia, Nitrite, Nitrate, KH (fresh or salt water depending on your setup)
If you can, get an electronic PH test meter.
Ammonia tells you if your filter is working properly. There should be almost no ammonia detectable.
Nitrite tells does the same. Different bacteria turn nitrite to nitrate.
There should be almost no nitrite detectable. This stuff causes brown blood disease.
Nitrate tells you if you are doing water change sufficiently
Nitrate naturally builds up in a filter based on aerobic beneficial bacteria.
It is controlled mainly by water change. It isn't very toxic to fish - but do not let it build up too much.
KH tells you if your water is adequately buffered agains PH spikes/dips
If KH is low, you will have to play chemist to add some buffer back into the water. For alkaline water loving fish like ranchu or koi, sodium bicarbonate is the preferred buffer. For acid water fishes, you will need to read up. I can't advise you on that. KH is consumed by an aerobic filter and will gradually be depleted. If it runs down to 0, filter performance will be inhibited. This is one of the most often neglected things in Singapore aquaria - especially so since our tap water is almost 0 KH unlike in many other countries. Coral chips and oyster shells can help to some extent. Remember though that these are slow releasers of carbonate (KH) and work only when the water is acidic.
PH tells you if you are maintaining your water at what's best for your fish
Fish like PH to stay nice and stable. Lack of buffer (KH) means a small amount of acid or alkali can cause serious PH swings. Swings mean death for fish. Commonly seen as a tank crash where all the inhabitants are found dead suddenly.