Sounds about right.
As you learned with the downward spiral in this tank, now when this occurs again, you'll have a better idea and add a little KNO3 and PO4 right?
My suggestion to add a pinch of KNO3/PO4 to non CO2 tanks once week will not hit a particular target per se, but it will relieve a fair amount of NO3/Po4 stress and keep the plants growing well.
Slow growing non CO2 tanks are much easier to dose and you can dose lower amounts without negative effects on the tank. Why? Uptake and demand are much slower
You have more wiggle room in your routine, so use that to your advantage!! Less work!
The point is through this testing you did, now you can better predict the future, I can because I've done a lot of testing and no longer need to test for many things, I already know and have tried various management methods that don't require testing.
Mid level experience aquarist are the worst and don't understand this yet.........but later they figure it out. The saying is " A little knowledge is the most dangerous".
You really did not need to test and you do not in the future.
All you had to do was follwo the advice I gave in the non CO2 method and watch the plants.
You know that as plant biomass increases, so will uptake and the the amount of nutrient removal. You also knew that the substrate was getting older and should account for that.
You very well could have predicted you needed to start supplementing with macros as well at that point when you saw the plants declining/algae apear etc, no test kit was needed.
Most low tech greeb tumbs do this, very few test.
Now what would happen if the test kit was inaccruate?
You would conclude it's not the low NO3/PO4 and assume thing s like alleopathic chemicals and Tom Barr does not know everything nor his behind from a hole in the wall.
That's occurred many time on the web
And it'll happen again........ nothing I can do about it either.
I think many assume I'm anti test kit, I'm not, I use them a great deal, but using them should teach you something an dyou shouild learn from the data and results.
Then you do not need them in the future, you have watched the plants and know the signs and what to do from then on.
Then you no longer require the newbie to tell you what their parameters are or not, just a pic or a good description of algae/growth and so forth.
Most can do that easily.
Then based of your knowledge, you can tell them to add a buit more KNO4/PO4 and do so once a week, just smidge etc and that will solve most of the issue for them.
Test kits are not items that should be used for a routine.
They are used to answer questions and can introduce more cost, work and assumptions that do not always solve the issue.
Intensive critical test can teach many things, then you no longer require them thereafter.
I work hard now so I don't have to later.
Same deal with the test kits.
Many folks simply will not test no matter what you say, most don;t want to test to begin with and no one wants to spend $$ of the test kits either.
So rather than sending folks down that path I took, I tell them not to test, watch their plants, spend their time working on cleaning/pruning./water changes/ and so forth.
For non CO2 tanks, adding a smidge 1x a week works well, larger tanks 2 smidges and so forth (smidge =~~ about 1 short rice grain). I just add abiout 5x less KH2PO4 than KNO3(eyeballing is fine).
This assumes you have a balanced fish load and good plant growth prior.
If you see a slow improvement, sometimes dramatic, that is good. If not, you likely have some other issue.
Most see improvements, a smidge of GH booster is very good and traces can also help.
If none of these help, then it's a filter, dead fish, too many fish, not enough plants etc.