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My big mistake.

Hi all,
I just recently had to correct a big error I made several months earlier in 
my 125 gallon tank.

The tank is an All-Glass 125 with 384 watts of PC lighting, CO2 injection. 
Heavily planted. I had a light colored gravel substrate in there originally, 
amended with laterite at the bottom inch. After a while I got the idea that a 
black substrate would be nice so I pulled all the plants and replaced the 
substrate with Top Fin gravel from Petsmart and amended it also with 
Laterite. This was really the only source of a very dark, consistent size 
gravel I could find.

All went well for a couple months. But then I noticed that my pH controller 
was reading in the low/mid 7s all the time, even at night, even after 
calibration (it was accurate). I also noticed that my GH/KH readings were 
slowly increasing, topping out a week or so ago at about 14 each. 

My tap water comes out with a GH/KH both less than 1. I have always had to 
build the KH and GH with chemical additions. I also have an automatic water 
changing system that does a 5% change daily on each of my three large planted 
tanks with tap water that has been run thru a GAC.

The tank looked nice, but the increasing GH/KH was bothering me and I 
couldn't understand what was the problem. I then noticed that the gravel I 
thought was all black was turning white. It turns out as Scott H mentioned in 
an earlier post that some of the Top Fin gravel brand is painted. It may be 
stated on the bags at the store, I don't know. I didn't notice it. 

I pulled a couple pieces out and crushed them. Sure enought, the gravel is 
made of a crystalline material that is whitish/clearish. Maybe marble or 
quartz or dolomite? Found the source of the buffering. Although the tank 
didn't look bad, I had had to increase the CO2 to the point where I was 
bubbling continuously (4-5/sec) into two Eheim canister filters I use on the 
125 just to keep the pH in the low/mid 7s, whereas before I bubbled into one 
Eheim at 2-3 bubbles/sec and maintained the pH below 7 without a problem. 
Apparently the coating was slowly flaking off, exposing more and more of the 
whitish stuff and increasing the surface area for dissolving the real 
material beneath the paint.

Anyways, had to bite the bullet and now the tank has a complete Fluorite 
substrate. No more pH problems and the bubble rate is back where it was 

I switched out my 65 gallon tank substrate at the same time as the 125 and 
added a much smaller amount of the Top Fin Gravel, and so far this has 
maintained a rock steady GH/KH of 5 without having to add anything, so no 
complaints there. The 125 gallon substrate was almost all this epoxy coated 

Live and learn, I guess. No more epoxy coated gravel for me. Maybe the acidic 
environment of the tank initially or the PC lighting degraded the epoxy.