Potassium test kit?

salifert is easiest. Elos and KZ are ridiculously useless IMO. I test a lot because algae scrubbers can pull down K also, as well as Zeo or any ULN system or so I've heard

 

Not really. But like any plant, N-P-K is what it takes up. N & P good. K dropping can be bad. Murray Camp told me to be careful though when messing with K levels. It is notoriously difficult to test for, apparently. The Salifert K kit is rather new, and seems to be pretty consistent. But I have had it give me some odd readings. MC said one of the dead giveaway indicators of low K is when the color of your monti caps (if you have them) go "dull". In the doc's tank I have a HUGE red cap and since I started dosing K the color has really gone vibrant - so much so I didn't realize that it was dull before!

Trace is a big gray area if you ask me. Lots of 'trace' supplements out there, I've read opinions that most of these are elements that are already in your system in levels that are much higher than NSW, some 100x or more, and I've also read that doing regular PWCs with artificial SW can keep increasing these levels, top this off with adding 'trace' supplements and this might actually be a factor for old tank syndrome.

Algae is known to uptake many different things, some metals, various contaminants, etc, but IMO this is barely studied at all in the aquarium industry - there's just no money in doing so.

 

Not nearly as complicated as the Strontium test kit. That one is a b-tch. Something like 10 steps, and takes about 30 minutes (including a 20 minute waiting period).