But the poster here is using CO2, thus the advice.
It's not a blanket statement, it's specific. If they had a different method, my advice would be different as well.

Still, many low tech folks never use a test kit and do quite well.

The slower plant growth allows you to see the plants as the "test kit".
Error for such non CO2 methods should always be in favor of running things "lean" for the plants and being able to top off once every 1-4 weeks with some inorganic ferts(or extra fish food in some cases, although I've had better results using inorganic ferts that have a better balance than fish food) for optimal conditions without test kits.

The idea is rather simple and it's very effective over years.

If you add too many fish, then you will have a gradual build up of NO3 and you do not have a balanced fish plant tank.

Now you will have more work, less success, more water changes, more testing etc. The goal of most low tech tanks is to avoid such work.

My low tech tanks never get any water changes, have a good amount of algae eaters, moderate fish loading, run slightly lean, I top off once a week with about 1/10th the amounts dosed in a CO2 low light tank.

If I see anything negative in the plants that suggest too little nutrients, I'll add a bit more ferts once a week.

If I see anything that suggest too much or I think I want to lean the tank up, I just no longer dose for 2-4 weeks till the plants suck up all the nutrients from the water column.

This method does NOT work if you add too many fish and over feed often. But then you don't have a balanced fish tank and that is a trade off you need to live with and adds more work for yourself.

If you want that much work and still demand that same fish load, just go low light/CO2 enrichment, that's not much work really and the water changes will help mitagate things a great deal.

Both methods do not require test kits if you set things up correctly and focus on a balanced tank with the fish loading for the non CO2 method.

It's not hard.
Some folkks add more food if the tank looks a bit "lean".
Some care and observations of the plants will go a long way.

Rather than suggest folks calibrate their test kits and go through all that, I'd rather see folks spend their time more productively in the hobby: not the "Test Kit hobby". Rather, the planted tank hobby, scaping, providing a good place for the fish, Gardening etc.

Many folks seem to think that everyone' AP test kit of their Hagen is perfect, they are not. So and so might have calibrated their one single test kit and it worked for them, but then another guy reads that and thinks they don't have because the other did the work.

That's a very bad idea and suggest that all test kits are equal, they are not.

If you do insist on using test kits be careful about assuming that they are right, make sure they are right, don't guess there and tell me I'm guessing with the EI dosing

I suggest that folks that want to test to get the Lamotte or Hach, better yet, the colorimeters.

This way you have good equipment and can do some decent data acquisition.
You will still need to know how to use and make calibration solutions, then make a calibration curve. You'll also need to know more about chemistry than many low tech folks ever want to know about.

I just tell them to add a pinch of this and that once a week in addition the feeding their fish.

This works very well.



Regards,
Tom Barr