City water departments try to keep the pH of their water above 7 at all times to avoid copper pipe erosion. If your KH and GH are both zero, your water is very likely to be below 7 in pH, so dissolved copper in the water is a real possibility. I believe some "whole house" water filters will remove copper, as well as other metals and other contaminants. I use one of those filters on my continuous water change water supply just to avoid that type of problem and to have a chance at removing most of the chlorine before the water gets to the tank.

Hi,
Sorry I wasn't specific enough. I am adding calcium carbonate which will increase both KH and GH as well as epsom salts for GH. Plus some baking soda too sometimes.

The tap water is 6.4 pH.

We do have a whole house carbon filter installed, but the problem is that the copper is coming from our house water pipes after the water hits the filter.

Will running carbon in my tank remove the calcium and magnesium that I am trying to increase, or will it affect fertilizers? Will copper harm plants? I know it harms some types of fish.

Thanks!

Yes, Copper is an affective herbicide/algicide. and running carbon in your tank filtration may effect your nutrient levels slightly, but so long as you are providing regular W/C's and EI it's a non-issue. As I stated earlier run the water for a few minutes prior to W/C's to flush the lines. To remove copper on a single pass you will need a solid carbon matrix ( not granulated) on your drinking water (IE: Multi-pure Filter). HTH. Prof M

Ok, I added carbon to the filter and I'm going to start adding baking soda, epsom salts, and calcium carbonate to each 50% water change. I started off yesterday adding about 1 tsp of each (baking soda and epsom salts) to my 4 gallon water change and it brought the KH up to 80 and GH up to 50 (both near 0 before). The pH also went up to about 7.6. Fish seem to be doing ok so far. The carbon didn't affect my KH overnight. I also put a few seashells in my filter too to keep the KH up. Not sure if I mentioned that I added two crypts and a vals so the tank is pretty well planted now. My lighting is still at 6 hours/day until I get my ferts.

How often should I be changing the carbon? And I heard once that carbon will leach phosphates into the water, is this true?

For high levels of copper I'd change out the carbon once a month at least. With your unique water chemistry I heartily suggest looking into the Seachem Onyx. It works very well for Acidic/Soft water conditions. I'd probably invest in a solid carbon drinking water filter too. They are about the most cost efficient option under the circumstances. Doulton is a highly reputable company up in the Michigan area. Activated Carbon Water Filtration HTH. Prof M