I must admit that I never paid a lot of attention to my water chemistry. When I first got a KH and GH test, I tested and noticed that they were both dangerously low (KH was 17.9 or less from my tap at that point). I added a bag of crushed coral to my tank, which kept the PH stable and brought KH up to 53.7, then I didn't think more about it other than to test my pH during a water change to make sure it held at 7.8. That was almost a year ago, and I've been thinking about it more as I have been managing an unfiltered hospital tank. I noticed that the pH of my hot and cold water is different, and I wondered if part of the reason could be that my cold water is filtered before the faucet. So, I decided to do more thorough tests of my kitchen hot water, kitchen cold water, the bathroom hot and cold water, and my tank. I fill my tanks from the kitchen. The bathroom water is unfiltered so it gives me somewhat of a "control" reading for the water in my house. I found that the hardness of my water has improved somewhat from last year, and that I should have been keeping a better eye on things.

Kitchen cold - KH 53.7, GH 71.6, pH 6.8
Kitchen hot- KH 53.7, GH 71.6, pH 7.2
Bathroom cold- KH 35.8, GH 53.7, pH 6.8
Bathroom hot- KH 35.8, GH 53.7, pH 7.0
Tank- KH 71.6, GH 89.5, pH 7.4

It's interesting that the hardness of my hot and cold water is the same for hot and cold at the same sink, even though the pH is different. It also varies slightly between sinks, but the filter does not appear to make much (if any) difference. I also noticed that tank temperature water, filled using hot and cold water from the kitchen, takes on the pH of the hot water at 7.2.

I haven't replaced the crushed coral in my tank's filter since I added it last spring. I'm so glad that I ran this test, because (coincidentally!) the pH has started to drop on that tank! Every time I've checked it before now, it's always been 7.8, although it's been a few weeks since I've checked anything but nitrates on that tank, I must admit. I just read about how dangerous a depleted bag of coral can be for a tank with low hardness, and that's certainly my case!

My plan is to replace the crushed coral on the tank at the next water change (tomorrow), then let it run for an hour and post the results. Then, I will need to do KH tests at least monthly, and pH weekly, to make sure I'm not headed towards a crash.

Does this sound sufficient?

Oh, there has been a change to my main tank since the last time I checked the pH- I added live plants. That wouldn't affect the pH, would it?

Ouchie, your kH and gH are low there. Similar to mine. The slight variations may be due to the level of the pipes/pressure but not 100% sure.

What I have done is fill the whole base of my tank with coral gravel as substrate. It looks really nice as substrate and has buffered my pH and raised my kH very well for several years with not a bump that I know of.

These are the coral stones I use as substrate

Thanks. I have a local pet store that carries several varieties of coral for their birds, and recommends it for goldfish when using our city water. I'll see if they have anything coarser.

I changed the crushed coral in my filter, and the tank KH and GH didn't change. The pH went back to 7.8, though. I notice that it goes up very quickly with baking soda, too. I have to use much less than the "recommended" amount to get the pH to climb. Do you know if the coral substrate would be harmful to plants? I'm thinking of moving my (now healthy) ranchu back in and letting her adjust, then adding a bit of coral substrate after I know she's adjusted to my current settings.