Not sure about this. Could be algae, but the way it looks to me seems more like a fungus. If you rub one off what is the texture?
A mystery snail might gobble that right up, whether fungus or algae.
It would be good to know the ammonia levels, as that's the part which can be the start of bad water parameters, illness and death.
Test strips also aren't really reliable. Even after double checking usually monthly, for many months, to compare the test strip reading with tests from the API liquid test kit, to make sure they were reading accurately. I had some test strips which killed most fish in a tank because the test strip wasn't reading the level at all, looking like the water was fine, when it was actually very toxic.
If you can get a LFS
to do a test to compare the reading, and take the water sample before you do a pwc
, and write down all the reading numbers they test for to kind of keep a log.
And consider getting a FW
master liquid test kit and read and follow the directions for shaking the solution.
Online values can be a notable reduced price. Also if you can see what the expiration date for the test kit is. It should be a 18 months to a couple years out. It was around $45 I saw today at the big box pet supply store and on amazon was around $21 usually around $25 with free shipping with Prime.