1/4 GH Booster is what I meant.
I never knew I was missing so many minerals that were in tap water.
1/4 teaspoon. It's the EI method for a 10g. I'm not sure how to measure degrees of GH but it's just a 1/4 teaspoon weekly.
It sounds like you're saying with EI dosing that it should be providing enough minerals on it's own.
Now tap water is certainly cheaper and easier, is there any downside to 100% tap with EI dosing?
I'm starting to figure that whether I used 100% tap, RO, or a mix that it won't be a big deal and impact anything too significantly. Which would lead me to start using tap for the convenience.
Dosing with RO
Super, now we have a rough idea about your dosing. A very rough estimate would be that 1/4 teaspoon in 10g would add a little less than 1 dGH, so very little but still something.
It is my understanding that the dosing for the GH booster was suggested as a supplement for water that is already remineralized. It should be added in the middle of the 'dosing week' in case plants use a good part of the available Mg, K, Ca . That being said, it is perfectly suitable to remineralize RO water, but you need to adjust the dosing. The GH booster is a ready made mix of CaSO4, K2SO4, and MgSO4.
To adjust KH you will need to add a source of carbonate CO3-2. Diana is right regarding KH-pH, but to me one of the advantages of using RO water is that you can adjust exactly what goes in.
GH can be measured using a cheap commercially available test, same for KH. You pretty much need it in the beginning until you figure the dosing,
EI adds traces and N,P, K. There are other macronutrients that are needed by plants C (from CO2), Ca and Mg are some. RO aims to strip the water of all other molecules, leaving you with almost pure H2O. It is up to you to add what you want/need.
RO or TAP
The correct answer as in most cases is that it depends. It depends on your tap water and what requirements your fish, shrimps and plants have. In some cases the tap water has high levels of contaminants such as heavy metals or other things that create problems in the aquarium. In most cases however tap water is ok to use.
Certain fish and shrimps require very soft water to breed and most people have medium to hard water ( at least in the EU). Certain people are just control freaks or run their aquariums like small experiments and what to be sure what is in there
Remineralized RO is a lot "cleaner"/simpler than tap water. Tap water may have traces of metals, NO3, PO4, etc (these can be good for your aquarium)
Bottom line, if you have no reason to use RO water and your tap water is good enough than don't bother with it, it is not a requirement of EI. The Estimative Index of Dosing, or No Need for Test Kits - Aquarium Plants - Barr Report
if you decide to go with tap, get the values of KH/GH from the water company. Depending on how high they are, you need to make small changes with tap water. A rapid change in GH/KH can KILL your fauna. Start mixing 10%-50% of your water change from tap and observe your fish behaviour. Also, if your tap water is soft, you might want to continue dosing the GH booster.