Tropical tank dropped by 6°C over 3 days

I switch off my internal heater at the wall every time I do a water change, because it will drop below the minimum water line. Usually, I turn it back on again immediately after the water is topped off, but I guess I forgot, because when I came to do another water change 3 days later (the only time I touch the water) I could tell it was cold. So cold that it wasn't registering the glass adhered thermometer.

I usually keep my water at 24°C, but it fell to the ambient room temp of 18°C, presumably within the first day. When I topped off the water (only a 10% change), I added slightly warmer water which immediately allowed the glass adhered thermometer to register its minimum 19°C. I was tempted to add very warm water to ascend the overall temp by more, but realised it would shock the fish too much.

I suppose I'm very lucky that the only animal species in my tank are relatively unbothered by low temps. The Celestial Pearl Danios and Amano Shrimp are both apparently fine all the way down to 15°C but my poor albino bronze corydoras are apparently only supposed to go to 20°C at the lowest.

All the inhabitants appeared absolutely fine! If anything, I was struck by their calm nature, not darting around as much. Now I obviously realise it's because their metabolism was slowing down. I noticed my corys were hanging around behind the internal filter, maybe because it was generating a small amount of heat.

I feel just awful, but I know I'll never make the same mistake again. And this is the benefit of doing more frequent water changes, I suppose. I've been doing two 10% changes every week, because the tank is still new.

I'm really just posting this for the purpose of recording my fishkeeping journey. Comments welcome, but not needed.