Good question/point. Here is my line of thinking:
These are the instructions for EI: https://www.plantedtank.net/forums/sh...ad.php?t=21944
*Dry Dosing Plantex CSM+B...
Converting 1 tablespoon to 250ml H20,
20 ml = 1/4 teaspoon of dry fertilizer.
There are 12 - 1/4 (1.08 g) teaspoons dissolved
50% H20 change-weekly
(1.3 g) 1/4 Tsp-KN03 3x a week
(0.35 g) 1/16 Tsp-KH2P04 3x aweek
1/2 Tsp-GH booster once a week
5ml or 1/16Tsp (0.27 g)-Trace 3x a week
1-2ml-Fe/Iron 3x a week
So according to these instructions you should be adding 3.9 grams KNO3, 1.05 grams PO4, and 0.81 grams CSM+B per week for a 20-40g tank.
Scale these instructions down to a 2-4 gallon tank and you should be adding:
0.39 g KNO3, 0.105 g PO4, 0.081 g CSM+B per week. Which works out to the same ppm concentration. The problem is, if you have the same plant mass in both tanks the smaller tank will run out of nutrients first because it has a smaller reservoir of each nutrient (as explained above). Maintaining constant nutrient concentrations in a small tank becomes much harder. As a nutrient is depleted plants start running into problems. To compensate for this you'd have to try maintain higher concentrations (to increase the reservoir) which is where the toxicity issue comes into play.
Also, most people cannot measure out tiny amounts of micros like CSM+B because they don't have accurate scales and don't know how to do a dilution. This means that there is an additional risk of overdosing micros.
Originally Posted by Zapins
There is that, but also if you dose a full week's supply of micros at the beginning of the week in a smaller tank you will reach toxicity levels.
I suppose saying you will [definitely] reach toxicity levels in smaller tanks is a bit of an overstatement, but hopefully you see what I mean from this post. More of a cautionary note to be aware of than a guarantee.
It is safer to space out the doses so you don't dump in a huge amount at the beginning of the week possibly cause toxicity issues, or have the plants run out half way through the week and starve for a whole week. Plants can get by with chronically low concentrations of nutrients if they are consistently present in the environment, but do not do as well in a feast or famine situation (dose at beginning starve the rest of the week). Better to keep consistent nutrient levels, which is why I suspect EI recommends 3x a week dosing rather than once a week which would undoubtedly be easier for the hobbyist.