My Corydoras are displaying some strange symptoms and I could use some advice. I'll give as much info as I can and then give a few guesses as to what's going on. Warning, this is a long read.
Some important context. My city tap water changed on the 1st of the month as part of their annual system flushing and this was my first w/c since that change. Per the city website; "normally uses chloramines, a combination of chlorine and ammonia, for the disinfection process. The State of North Carolina encourages all water systems that add ammonia to cleanse their system annually by switching to chlorine only and flush the system"
I have 2 (1 passed tonight 3/11) Corydoras paleatus females around 6 years old in a 20 gallon tank with 15 cherry barbs of varying size (None of the barbs are showing any symptoms or strange behavior.). Did a 30-40% water change Thursday night and by Friday midday 2 of them started acting shy, not searching for food, rapid gill movement and trouble orienting themselves when swimming progressing to twirling in circles. They are darting to gulp surface air a normal amount (as most Corys do) They could have been displaying some of these symptoms earlier and I didn't notice, but not much earlier. Max 48 hours. The one female is not displaying any symptoms and is acting normal. She has always been the largest and healthiest. All of these symptoms have gotten worse friday night and saturday. I added an airstone to offgas any residual Chlorine on Thursday night after water change. One female passed and the other sick one is looking on deaths door lying on her side on the bottom w/ continuous gill movement. Heavily considering euthanizing her. I have good video of symptoms but don't know how to upload a video.
They have no cosmetic signs of disease whatsoever. Very good coloration, plump, healthy barbels, no streaking or discoloration of fins.
On Friday night I reached out to a catfish keeper in my Aquarium club. She suggested I do a large water change with storebought spring water. I did a 60% water change with remineralized distilled water because that's all I could find. 2x dosed prime with this waterchange. Added several square inches of Polyfilter. No improvement.
This tank is slightly overstocked but I've found the water lettuce under high light scrubs the water very well and I remove several handfuls of biomass every few weeks. Hence why an ammonia spike seems unlikely. Nitrates never get above 25.
Tank parameters: 20 gallon long tank 3 years old with sponge filter. Lots of water lettuce and water sprite. 1/4 inch pool filter sand and some leaf litter (magnolia and oak) pH 6 GH 3-4 KH 0-1 Ammonia 0 Nitrate ~25 Nitrite 0 Temp 71F
Tap water parameters as of 3/11 pH API says 7.6 or higher. Co-op test strips read 6.4 so not sure what that's about. GH 3-4 KH 2-3 Chlorine at least 1.5 ppm
My current theories are; 1. There somehow was residual chlorine in the water even after aerating 3/4 of the water for 24hours and double dosing prime. A few of the gallons I added during the water change were not aerated overnight but still dosed with Prime. I've read Corydoras can be especially sensitive to chlorine. 2. A slight ammonia spike from water lettuce root dieoff (which seems to come in cycles. They grow long and then die off rinse repeat) or built up mulm. Don't know how this is possible considering the plant mass in the tank and it hasn't happened before 3. A pH crash from built up mulm/leaf litter and a lack of kH. I admittedly don't understand pH very well but I've always been paranoid that with my low KH water and use of leaf litter, I might experience a pH crash at some point. I should probably start using crushed coral like Cory is always recommending. 4. Parasites. I lost 3 male corys 6 months ago. I'm pretty sure they were outcompeted for food but I always suspected they had parasites. They were very bad at eating compared to the females. Has anyone had this problem before? They lived for 5-6 years but were always borderline apathetic towards food. 5. Disease. I don't know of any disease that would kill so fast and manifest NO cosmetic symptoms whatsoever.
So that's it. I love these fish and it's very discouraging to lose them. They seemed so healthy. Any help is appreciated!
