Follow up question... I got the handy - dandy dosing calculator and I see how it calculates the 4 teaspoons for a 180 g tank as 20 ppm . ( tool recommends 5ppm nitrate .. whatever ... that s another issue ... ) . So if you dump in the 4 teaspoons, how do you know what to dump in tomorrow / next day / next week.
If these API testkits are as crappy as I've been reading, is there any way to gauge if the nitrates are leveled off at 20ppm throughout the week? Or do you just assume that the plants are burning up entire dose every day?
You can NOT assume that your plants use up 20ppm every day. In other words, don't add that much to your tank daily, unless you want to turn it into a brackish setup.
You can make some assumptions. If you know the NO3 levels of your tapwater, you can add a certain amount to that after doing a 50% water change. Let's say 4 tsp bring your tank to 20 ppm, and you found out that your water contains 0 ppm, then you could assume that adding 2 teaspoons would bring the tank to around 20 ppm if your plants didn't use anything, or a bit less if they used up some of the fertilizer.
Stocking levels, cleanliness, filtration and such play a role too, main things that affect nutrient uptake is light, availability of other nutrients (CO2!), and types of plants and plant density. Each tank is different!
Instead of assuming, you could get a test kit, calibrate it somehow so you get better/more reliable results, and learn how much nutrients your tank is using. After a few months of first frequent testing, later once in a while, you can retire the test kit.