I seem to recall reading somewhere that slate raises hardness and pushes the pH towards alkaline levels. I read here that slate is inert. What's the truth? I am confused?
Slate Raising GH/KH
- Thread starter Radek Z
- Start date
Forgot to say I have a lot of slate in my tank and I seem to be suffering from hardness creep - I change water and it's nice KH 5 and three days later I am hitting KH 8 or 9.
Slate is not a carbonate rock which means it will not precipitate out any minerals that would affect your hardness. My guess would be that your water has a high mineral content and that's what the cause is. Rocks that would affect your tank would be more along the lines of limestone, marble, calcite etc.. If you're ever in doubt about a rock, pour a dilute hydrochloric acid solution (usually 10%) onto the rock to see if it fizzes or just try the kitchen version: vinegar. 
OK, in the meantime I found another mention of slate as a buffer on cichlid-forum.com. May be I am missing something.
At the moment I have granite, slate in the tank. I have taken out one strange rock I could not really identify (close to granite but a bit lighter). Will see if that makes a difference.
The mineral content in the water should not be an issue as I am adding RO only. Normally I remineralize this to KH 4 with RO additive. For last few changes I have added RO only and still got KH creeping up.
Anyway, it's not a big issue - easy to keep under control with water changes. I will play with taking some rocks out and see if I find the culprit.
At the moment I have granite, slate in the tank. I have taken out one strange rock I could not really identify (close to granite but a bit lighter). Will see if that makes a difference.
The mineral content in the water should not be an issue as I am adding RO only. Normally I remineralize this to KH 4 with RO additive. For last few changes I have added RO only and still got KH creeping up.
Anyway, it's not a big issue - easy to keep under control with water changes. I will play with taking some rocks out and see if I find the culprit.