YellaFox

With this tank I tried something new, using Activ-Flora substrate, which (supposedly) provides a very rapid start. Dosing with a bit of fish food, I did see low ammonia, then trace NO2, NO3, but you may be right with the fluctuations. I've been trying to test every day, maybe will add a 2nd test to see if it's swinging. Will also look into KanaPlex. @Littlefish Yes, GH & KH are ppm. Municipal water source is super soft, I added several ounces of crushed coral to the canister to boost minerals. @Tankseeker Definitely -- though am a bit hesitant to add too many things at once 😅

Yup, same place (Aquarium Co-Op). All fish looked healthy & fit when they joined the tank, don't think the source is at fault. My canister is running just shy of 150g/hr, media volume is 2+ liters, practically as much BioMax as I could fit. Current population is 26 fish. Interior tank flow is gentle ... but maybe we've got too many water changes per hour? With overstocking, I would usually expect to see degraded water params, which I'm just not getting 🤔

Washington, near Lynnwood. Aquarium Co-Op is my LFS 🙂

Basics 17G freshwater planted tank running about 18 days, midsize canister filter, w/ Ziss air stone, in-tank heater. Water clarity is good, no "pond" smell, tank interior seeing respectable algal growth (green dust, spot, brown diatom). Reducing lighting duration to combat, no fertilizer or CO2 in use. Temp: 74F PH: 7 Ammonia: 0 Nitrate: 0 Nitrite: 0 GH: 30 KH: 40 Hardscaped, planted, and ran tank w/o fish for the first week, testing each day. Confirmed stable, added 5 Red Phantom & 5 Cardinal tetra around day 10. Zero issues, fish displayed healthy color and relaxed schooling behaviors. Next day, added 5 panda corydoras. Roughly 8 hours later one began showing swim bladder / coordination issues. Found dead later that evening, maybe with slight film on one eye? Inconclusive. - Next day, another panda down, filmy eyes. - Next day, another panda down, no visible issues. This past Friday evening I noticed both remaining corys had visible white cottony "dust" indicative of a bacterial infection. Saturday AM I picked up Maracyn on the advice of the Co-Op crew, begin dosing for the correct water volume. I am now consistently losing one fish per day, with the following sequence: - Visible loss of coordination / fine motor controls, with symptoms worsening over several hours - Capture affected individual in fine net, suspend in gentle return flow to ensure available oxygen - Death It is now affecting all tank species (except the Amano shrimp), BUT my two corys remain hardy! Signs of infection are diminishing. I am hesitant to change the water, as that will affect the Maracyn protocol, and am completely baffled. Since the weekend each lost fish has zero visible stress indicators / no signs of illness / good color, until rapid decline sequence outlined above. I have also considered adding UV to the return loop, but the Maracyn instructions advise against it. I've attached pictures of our most recent loss (the white things that look like teeth are bubbles). This feels like an issue with bacteria suspended in the water column, but the onset of illness seems quite fast. Ideas?