I accidentally temperature shocked my fish (I think)

[Kat_Rigel]

I have been fighting "old tank syndrome" and have been doing regular 30-40% water changes one a week for the past 3-4 weeks in my 55gal tank.

Today I changed my water as usual, but my kyathit (orange fin) danios are showing a LOT of stress! The guppies and white cloud minnows look fine, but the pencilfish are hiding instead of swimming about. I found one dead danio, which was obviously a red flag. It was clearly only dead for a very short amount of time, no decay, and no obvious signs like scratches, ammonia burn gills, pineconing, etc (Gills were not moving; I am certain it was dead and not just shocked.) I found a second one flailing and in death throes. I panicked and threw it into the nearby, much warmer quarantine tank (QT is almost done for those fish anyway.) It looks like it has recovered a bit, as it is now upright, but it is clearly still stressed because it's gill movement is very fast. Plus, these danios are usually zipping around, all chasing one another. I can't even see the remaining 3 in the main tank; they are often hard to find in my heavily planted tank.

I have checked the heater and found some condensation inside; I removed it (it will be retired now) and place a backup heater.

After water change parameters, according to Tetra test strips, are: 68*F (I think it got down to 63/64* It was an accident! I used water that was too cool. It was such a big water change that I set it and started working other tanks rather than monitoring it constantly.) It is usually 70*. Nitrate between 20 and 40 (the parameter I have been struggling to lower; it was up above 80 3/4 weeks ago!) Nitrite is zero, hardness consistent at 300+ppm, zero chlorine (I used dechlorinater as always,) alkalinity 120ppm (consistent), pH 7.8 (also consistent.)
 

First, am I missing something obvious that could have stressed them? I recently checked the tap water parameters and they were all reasonable (zero nitrate, nitrate, ammonia.) Should I do anything else to help the fish recover other than correcting the water temperature? The new heater is smaller (it's a backup heater after all) but it's chugging away- I am keeping a hawk eye on it to make sure I don't cause a temp swing in the opposite direction.

And tell me it's ok to make a mistake, I'm so upset with myself! 🤕😭 😭 

[Alison]

Its ok to make mistakes. We all do now and then. I know it doesn't make it feel any better but you are definitly not alone. I left my heater unplugged for almost a week after doing a water change and I did lose a couple fish to it. Just gradually raise the water temperature slowly until it's back where it needs to be.  I know how tempting it is to correct it immediately but that just adds to the shock. Keep us updated on how it goes!

 

[Tanked]

I've been there and done that, but without the fatalities.  Last winter I realized that my Tinfoil Barbs had stopped coming to meet me when I entered the room.  I had unplugged the heater instead of a malfunctioning powerhead.  The water temp. had dropped to the room temperature of 60 degrees for several days. 

My solution is to keep water preheated with a spare heater for top ups or changes.  I don't know if is efficient, but it is a time saver.

[Kat_Rigel]

Everybody is looking better today, I got the temperature back where it's supposed to be, and pencilfish definitely have their swagger back. I keep seeing only one kyathit danio, but as I said previously, they are tough to see in general. Plus it would make sense that they take more time to recover, since they were the most sensitive initially. Everybody else is acting like nothing happened, so hopefully the damage was minimal. Thanks for the support, guys! It's so gut wrenching when you mess up!