Completely forgot to add info about my tanks, so I'll do that now.
The main tank I got from my friend is a 30g freshwater, which originally came with 2 orange koi angelfish, a single black neon tetra, a scissortail rasbora (schooling with the tetra), and quite a large common pleco. I had to get rid of the pleco and add more tetras to increase the school. I also added a small blue gourami early on which probably wasn't the best decision, but he gets along fine with his tank mates so I'm not too concerned.
After about a year of having them, most of my fish suddenly died off one by one. I went from 11 to 2 in the span of a week. Despite all kinds of tests and research, I could not come to a clear conclusion about the cause. This initially made me lose hope in the hobby. However, I decided to take this as a learning opportunity and start again. I've since improved many aspects of my aquarium keeping habits, hoping to avoid any more future disasters.
Currently, the two in my main tank are the blue gourami and a lone scissortail rasbora. I set up a 10g quarantine and have 4 new rasboras waiting to be transfered. I also got an otocinclus to take care of my moderate brown diatom issue in my main tank, but I realized afterwards that they need to also be kept in multiples. I will be returning to the LFS to get more soon.
