this thread is a little confusing, maybe step back and answer a few questions to better focus your thinking.
1) what is the problem here?
- if your issue is that you got a number you didn't like on an unreliable test, start there. Confirm the test.
-if your issue is the algae, start there. Nitrate is part of it, but not all. But the first thing to do is still get a legit reading. You'll also want to test phosphates, and the API test is not good enough for that because it doesn't go low enough (I find the API nitrate works fine, but lots don't).
2) what have you done already? You mentioned a few buffers and "nitrate blockers" please describe in more detail other fixes you have tried.
3) what is your current nutrient export, and import? It sounds like you don't have a skimmer, and you do regular water changes, and you are (maybe we're) running carbon. How often do you feed, what, and how much?
Other thoughts:
That's a decent number of fish for this tank, subpar nutrient export is going to result in algae because you simply must feed enough for the fish to live. You must then remove those nutrients.
That looks like some nice Pukani rock, you could probably use more of it and I would guess it is leaching a lot of phosphate into your water, fueling algae.
This will go better if you slow down and make careful, researched choices about what you do to the tank. Like Heathlinder said, adding chemiclean would have been a mistake, and it could have made things much worse. You are not in a crisis here at all, you have normal boring new tank problems and you will most likely fix them with normal boring solutions. That's really not so much algae at all.
