Do you add CO2?
My understanding is that if you don't, the plants will have difficulty taking up any added ferts.
I went thru a frustrating time re: test kits. Couldn't find any PO4 at LFS or the big boxes. Then got a NO3 and Ph which I didn't like. Couldn't decide about Kh/Gh. And the packaged kits had a bunch of stuff I didn't need. And it can be a hassle running around from store to store trying to compare.
Realize you need to test both NO3 and PO4, cause the respective levels don't matter as much as the ratio between the two. I strongly suggest identifying the tests you want, and buying them all on-line in one shipment. Most people have good things to say about the Red Sea kits. It really sucks, if you don't know much about the chemistry, to go out and spend $10 on a test kit, and use it once and find out you don't like it.
Now I do little if any testing, tho. I was fortunate enough to find a local planted tank club that all use the same water as me - from Lake Mich. They impressed upon me the need to start CO2 first, before worrying about ferts. (I was having algae problems at the time as well.) They gave me some powder NO3 and PO4, and I picked up some TMG.
Consider getting some fast growing plants - anarcharis,hornwort, hygro, etc., to outcompete with the algae.
Now my plants are going gangbusters, and the algae is long gone. Maybe I'm living on borrowed time, but I'll get more into testing if things crash.
Just wanted to toss in the possibility of getting some decent results "playing it by ear," instead of worrying overmuch about nailing down the chemistry.
Also, IMO the folks on these boards tend to go a little heavy on the water changes. While major water changes can't hurt, they may not be absolutely necessary. I do approx 20% on Sat, and 10% on Wed. and it seems to work out fine. Of course, I'm schlepping buckets. But if you have a python going, no problem running it a little longer.
Just my $.02 (and probably overpriced at that!).