Carbonate is used as part of the aquarium cycle which will lower ph/kh. At a ph below 6.5 the bacteria slow down and at 6 they get dormant. So at 6 I'd say your bacteria are not being effective.
Baking soda (sodium bicarbonate) is a quick fix but can change ph very quickly so you don't need much. Crushed coral or cuttlebone (calcium carbonate) in the filter is a more natural, long term fix and works well at a ph below 7. I have some crushed shells in mine as backup.
If you check tap water (let stand for 24hrs to degas, etc) you will get base ph and in a lot of cases, just increasing water changes will fix tank ph if your tap ph is above 7.
In my case over summer tap ph is naturally lower plus warmer tank plus larger fish meant my ph was dropping quite a bit.
Edit - just jumped on the desktop and below is a good article on heter. bacteria.
http://www.oscarfish.com/article-home/water/72-heterotrophic-bacteria.html