GH of tank water >> tap water. Solutions/suggestions?

[Ganthet]

Hi everyone,

For the first time in all my years of maintaining aquariums, I decided to test the GH and KH of my tank water with an API test kit. I was very shocked by the results, as the water in my tank is significantly harder than my tap water. The results are as follows:

Tap water
GH 4-5
KH 2

Tank water
GH 8
KH 2-3

As you can see, the difference in GH is too large for it to be a coincidence/error. This is despite the fact that there are many pieces of bogwood in my tank (no rocks), and we also dose 5ml of blackwater tannins every week. Seachem fertilisers are added at half doses, not enough to change GH or KH.

I personally suspect that the water hardness is due to my substrate (we use a mix of gravel and soil). Both were purchased from Nanyang Seaview. When I look carefully at the gravel, I can see small bits of crushed shells, which might have been buffering the hardness. Here's an old FTS of my tank.

Does anyone have other good suggestions on possible causes? Suggestions on how to lower GH would be appreciated as well.

[shoelevy]

a quick search on google and i found hobbyists with similar problems of high GH/kH/pH from using gravel. before using ADA aquasoil, i was using sand and had the same problem where tap water chemistry milder than water in the tank. As soon as i changed out the sand, i had much better numbers for plants and related lifestocks.

but if you say you've been doing aquariums for years and i don't suppose you have issues with your lifestock, better "not to fix sth if it ain't broken".