Back with some update.
I have removed the decoration just in case and put a PVC pipe in there in its place.
The Angel doesn't seems to be too bothered by that.
I still haven't been able to make the waterchange that I desperate need.
I am having some problem with the pH and Alk of the newly mixed water.
I spent the morning reading a couple article on pH, Alk, Calcium and Magnesium and, though I suspect I have a magnesium problem, I'm still not sure what the problem is.
I'm hoping maybe a can get some tip from here.
My waterchange water usually start with pH of ~8.0, Alk of ~8dKH, and Calcium of ~420ppm when it is mixed to the SG of 1.024.
I usually use baked baking soda to boost the pH and KH higher before I use the water. I usually aim for pH of 8.2~8.3 and 11dKH.
2 nights ago, in anticipation of the QT waterchange, I checked the parameters of the 10gallons of 1.024 water that I have been mixing for close to a week already. (I wasn't going to use all so don't worry, will explain later)
During that time I didn't have my pH monitor with me so I only checked SG and Alk.
The SG was 1.024 but Alk was only 8dKH.
I was puzzled that the Alk is still at 8dKH because I have been adding about 1teaspoon of baked baking soda per day for around 2 days prior to the testing.
I thought maybe I needed to add way more so this time (2 night ago) I dissolved about 2 teaspoon of baked bakng soda in about 4~5 cups of water and added it directly into the 10gallons of water.
I came back last night and checked Alk again to find that it is STILL at 8dKH.
By last night I have already fixed my pH monitor so I gave it a check and was even more puzzled to find that the pH has dropped to 7.8.
I also started seeing some light precipitation forming on my heater and pump but the water seems otherwise clear and I couldn't feel much precipitation on the bottom of the container.
My question is, can I add too much baked baking soda to make it work in the oppsite way? (lowering the pH and not increasing or even lower the Alk?)
I wanted to know because I wanted to use ~3 gallons of these water to do a quick waterchange for the QT and raising the SG a little bit at the same time (I did this before and this volume with this SG should raise the QT no more than 0.002 SG)
I have taken quite a few chemistry class and read a couple of articles on this, including those Lee suggested, but I just never seems to be able to fully understand and control this pH and Alk problem and it is getting extremely frustrating.
I'm heading out to try to find a magnesium testkit now and see if that is the problem.
Hopefully I can figure this out quickly so I don't delay the waterchange any longer, because the Angel is still not really eating anything, though still active.