I'm still having issues with the pH in my tank fluctuating like crazy. It was fairly stable for a few days around 6.8-7.0 and then yesterday it was up to 7.4 and today it's down to <6. I didn't do anything to the tank between those two readings. Any idea what's going on and what I can do to stabilize it? I have a hunk of base rock in the tank and had been adding baking soda periodically to bring the pH up but I haven't done anything in a few days because it had been stable. I just did a ~50% WC
which brought the pH up to about 6.6, which I know is a big change for the fish but nitrite levels had spiked up to dangerous levels.
I was also wondering if the brown gunk on my air line and in my filter is algae and what I can do about it. It doesn't really bother me that much since its only on the air line and in the filter but I don't want it to take over the tank or anything. Best pics I could get are below.