Growth on mouth

[Kmorpeth]

1~ My angelfish has a growth on its upper mouth, it appears to have grown in A few days. It appeared cauliflower like and transparent, then appeared to have bled under the skin and is now a purple color. Other fish in my tank have recently died. I lost two neons and a glow tetra.
2~My tank parameters are ammonia, nitrates and nitrites 0 and ph 8. My kit did expire though and I have ordered a new one to recheck. I have also since added ph down and some driftwood and peat moss to help my angel. My tap water is a ph 8 as well. Water is 80 degrees
3. My tank is a 36 gallon. It is a bow front. I have had it running for 4 + years
4~I have a marine land penguin bio wheel 350
5~ Now I only have the one angel (size of my hand including fins) and one glow tetra left. Glow tetra has a white ball on right fin.
I have had a large angel with a growth on its mouth years ago and it died a couple months later.
6~Yesterday (after testing) I did a 30% water change. I admit it’s been a month or two since the previous.
7~ I have had all fish over a year.
8~ Today I put in sand substrate (changed from gravel) and amazon swords To help my angel feel more comfortable. I also started treating water with pimafix and melafix. I have purchased Furan-2 but have not started treatment yet as I am not sure if this is indicated.
9~Fish still has a great appetite. I feed tetramin plus tropical flakes. I just offered some blood worms for a nutritional boost.

 

[Autumnsky]

Lymphocystis disease is common in fish but is most often whitish.

The dark color makes me think of melanoma. Try checking these things out and see which looks closer.

By changing out the substrate you may have a mini cycle due to removing some BB.

I would do a few more pwc soon. If you were just adding in more tap water (dechlorinated) when adding water due to evaporation, you have been compounding the TDS total dissolved solids and likely putting extra stress on the fish.

Mineral and solids stay in the tank and water evaporates. You have high pH from your tap at 8 pH, if I understood that correctly.

You can use RO or distilled water for the evaporation to help not have additional build up.

Did you get your new kit and what are the new kit water parameters? Did you try using the high pH to get a more accurate pH number?