Help me understand water hardness

Those are strange readings. I'm wondering about the accuracy of your test kit.

I could understand water getting harder if you had interesting substrate that was disolving, but softer in the established tank really puzzles me.

Do you get the same readings if you repeat the tests?

Multiple times. I didn't trust the test either - so I ran it against some known quantity water. It's accurate.

The thing that makes my established tank test odd is that NO amount of solution changes the color (this is an AP test kit). I looked this up - and it said that this means 0 on both the KH and GH readings. I've never paid much attention to hardness - so I have no idea if this has always been the situation.

I can sort of understand the slight bump in the new tank. My substrate is eco complete - and I've heard that a few degree increase is to be expected but it will fade after a few months. But I'm stumped on the hardness fade in the established tank (the one I ran the test on multiple times). I mean - doesn't this indicate that something is leaching into the tank? The only thing in there is an Anubias and a terra-cotta jar and some cheap plastic plants! Does KH (and GH for that matter) naturally fade?

kh will decline given enough time and fish poo/organics. How often and what percentage are your wayter changes? Using different water, peat, or a water softener pillow are the only ways I'm familiar with for lowering gh. I'm interested to see what others might think would bring the gh down.

On this tank - I do about 20 percent water changes twice a month. I didn't realize kh could drop - perhaps I need to do a better job vacuuming? Which leads me to my second point...

I started thinking about the gh and wondering if the test was actually accurate. So what I decided to do is find out if maybe the gh was actually crazy high and the test wasn't reading it. So I started diluting the tank water used in the test with higher and higher portions of my gh 5 tap water until I got the test to register. Turns out, the gh is WAY up in the tank - its actually 30 gh!

So I have a 0 kh (which I verified with the same method I used for the gh) and a super high gh (relative to the tap water). So does this just mean my tank is a mess? I'm pretty good about keeping clean. Or could it have something to do with the fact that I 'top-off' this tank fairly often? I lose about 2 gallons a week to evaporation on this tank and I usually just top it off. Could this be doing it?

If your kH was 0 (or extremely low), the test solution would change to the end color with the first drop. Is this what is happening?

Top offs with tap water will increase hardness over time (assuming none is consumed, which is a safe assumption). Evaporating water leaves minerals behind, top offs with tap water add more minerals. Rocks and other decor that leach minerals can also exacerbate this.

Yes. That's what is happening for KH. GH is different though.

Sounds like "old tand syndrome", or at least the first symptoms of it.

I think you are correct. I just did a nitrate test and that too is quite high. I guess I don't do big enough water changes. I'm going to do some daily 10 percent changes and see if I can slowly get this tank back down to normal. I guess it pays to actually run some tests once in a while