Ok first off you will most likely have issues with 4 clowns in a 20g. They don't play well together. I had 4 in a 6 foot tank and after a month or so the one pair almost killed the other 2. It usually dosn't work in a big tank so your chances are worse in a tank that size.
Ok, as far as testing, ditch those test strips, pretty much useless. get an API liquid saltwater master kit for like $20. It is way more accurate and cheap too, you get all the tests that you need.
I would change out 4g-5g a week or so, it's quick enough to do and will keep your tank in good shape.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.