Els Engel:
Hi all, we measured all the systems today, and KH is back to normal everywhere. GH is very high though... it's between 20 and 30 in all the systems. Does that have any implications?
It is good that the KH is lower now. The fish will not suffer CO2 stress now.
Most fish and plants like lower GH's. GH is a measure for mainly dissolved Ca++ and Mg++. There are some possibilities why it got high:
Evaporation: The dissolved ions do not evaporate, so they concentrate in the remaining water. Every time the evaporated water is refilled, extra ions are added to the mixture.
Something gives off ions: This could be the fishfood, the pebbles, lime or something else that was added to the system.
To lower GH you can do water changes. The GH in the tapwater is 8, so you would have to change large amounts. You could also use rainwater or reversed osmosis (RO) water. Those have a GH close to zero. If you use rain or RO water the water changes would be more effective.