[rhodesengr]

OK so I have been using RO/DI water for the last few months on my two tanks: 36 gallon tropical and 26 gallon goldfish. I was keeping the GH at around 3-4. I went on vacation for two weeks so it had been just short of 3 weeks since the last water change. I found the GH in both tanks was about 10. Nothing was added except fish food.

Can the GH increase almost 3-times just from fish food and fish waste?

 

[Darkblade48]

The GH will not increase due to fish waste/fish food.

I would suspect your substrate/hardscape that is leeching minerals that is contributing to the increase in GH.

 

[rhodesengr]

I have red Florite substrate and a couple pieces of drift wood in each tank. Is Florite known to leech Mg and Ca into the water?