I use a simple API 5 in 1 test strip kit. I expect you to now tell me it's useless trash and I need to upgrade to a better kit for reading nitrates.
No.. I don't care what test kit you have.. when it comes to nitrates, all test kits are trash until proven otherwise by checking them against a known sample...
I personally have yet to find a strip test that I trust, but I'll admit I've not tried API's. I use their liquid tests, and most seem fairly good, but my set of their liquid nitrate test reads 6 times actual levels.
My first hint that it was off was it told me my tap water was 20ppm, which exceeds legal limits (it's actually 3) ... I then mixed up a some reference solutions and found out it was reading 6 times too high.
I am by far not the only one that has had this problem..
Also that result doesn't make the test trash. The results I get from the API liquid test are *very* consistent... I just need to divide its numbers by 6 when I use it.
I now use a different test most of the time, as it is easier to read in the 10-20ppm range than the 6x API test... but I still keep the API one as it is useful for measuring very low values.
Moral of the story: Don't trust your nitrate test without reason to do so. It is very common for them to deceive you.
On the other hand I did pick up some API root tabs today. I also have Jobe's plant spikes which might be a tad heavier in nitrogen but are even worse than root tabs when it comes to disintegration upon contact with water being that they are made for terrestrial plants. At least with the root tabs I have a fighting chance to get them somewhat buried if I prep a hole and act quick. Still not something I like to do when my substrate floor is barely visible.
That's a good step.. I've used the API tabs and seachem tabs before.. the API ones are NPK with iron, but dissolve too fast, and the seachem ones have lots of micros, but don't have enough NPK. They both seem to work fine for what they contain..
I now do the DIY osmocote+ thing, and like those a lot. However, those have their own problem of leaving little beads behind in the substrate. This isn't really an issue if you bury them really deep, but when I pull up plants completely to rescape they end up popping up.
I also do the "mix my own liquid fertilizers from dry"... it ends up being pretty cheap that way, I'd suggest considering that option as well.