I'll try to keep this short and to the point. Thanks in advance to anyone willing to offer advice.
40 Gallon tank
canister filter with spray bar facing glass and one half of surface. half of the surface is relatively still for floating plants.
79F
heavily planted with water sprite, hygrophila siamensis, java fern, anubias, pennywort, frog bit, & of course duckweed,
1 crowntail betta, 8 Harlequin Rasboras, 6 Kuhli Loaches, some amano shrimp, 2 nerite snails, and a handful of small snails.
current parameters (using aquarium co-op test strips
hardness 75-150 ppm
buffer 40 ppm
pH 6.8
Nitrate 25 ppm
Nitrite 0 ppm
Ammonia 0 ppm
Story
Friday 10/29
while feeding the Betta he I noticed his tail had black fungus on it. I think it matched tail rot pretty well after research.
I moved the betta to a 3 gallon plastic container with a small filter and heater
I added 1 tablespoon of salt (per this article https://www.aquariumcoop.com/blogs/aquarium/aquarium-salt-for-sick-fish)
Cleaned the 40 gallon. (scrubbed glass, cleaned filter media, trimmed and pruned plants & 25% water change)
Saturday 10/30
fed the betta one pellet (xtreme 1.5mm)
mixed up three gallons of water with conditioner and salt in a separate bucket to use for water changes
changed 1 gallon of water (1/3 total)
Sunday 10/31
fed one pellet
changed 1 gallon of water
Monday 11/01
fed one pellet
changed 1 gallon of water
Tuesday 11/02
fed one pellet but did not eat it. (I removed the uneaten pellet)
changed 1 gallon of water (mixed up another 3 gallons of salt water)
Wednesday 11/03
I tried to feed him a single flake of fishfood but he didn't eat it.
changed 1 gallon of water
I determined the fin rot wasn't improving so I added 1/3 packet of API EM Erythromycin.
That night I noticed he was curled up on the bottom. (He usually chills near the surface)
Thursday 11/04
He was still curled up in the morning. I poked him and his body was stiff. dead.
He was a pretty active fish until I added the E.M. Erythromycin. Did I add the wrong amount or do something wrong there? I can't figure out where I went wrong. Was I not supposed to use salt and the medicine at the same time? I'm feeling a bit discouraged and don't want to rush to replace my centerpiece fish until I understand what went wrong.
