They sell such teaspoons, Wal Mart etc, you can also do the divide method, take a volume of 1/4, divide it by 2, 4 etc.
Your eye is good at 1/2's, not 1/3rds.
Then you have a close approximation.
However, it's generally best to use liquids and make a stock solution if you have tanks 20 gal or less. If your dosing is off 2X with PO4, no big deal, NO3, you likely will want more control. Since we add less PO4 in general, those low ranges really affect PO4 mostly, not NO3.
As long as you have 0.5 to 3.0 ppm or so, you are fine. If you have 5ppm, that's fine etc. Just be consistent in dosing.
Liquids or powders.
Overall, for the non CO2 method, liquids are a better option since you need less.
But you also have much much larger wiggle room.
You do not need the precision you might want with a high industrial growth system that adds CO2
So do not fall for the BS some will try and argue, the non CO2 system is much different and the response times and intensity of limitations is far less.
Nothing wrong with more precision, I just saying it is not required.
Wait a week or two longer(skip a week or two of dosing) to remove what's there if you over dose, feed the fish a bit more if you under dose etc.
The main reason is to just prevent strong limitations that will cause some species to not do well ands to relieve any plant=plant competitions(just like in natural system that don't have enriched CO2).
Plant - plant competition does occur in non CO2. I think folks do not see or realize it. We see it a lot in natural systems and it helps to explain their Ecology.
It can also occur in CO2 enriched systems as well.........some plants are very aggressive growers and will mop up all the CO2 or be able to start removing it much faster than another species.
In such tanks, you see some plants doing great, but a few not doing well over time. Adding say some more PO4, often relieves the issue, or more CO2 etc.
As biomass increases, this can occur more rapidly.
Regards,
Tom Barr