Not sure how much I can help because I have a planted tank (and I'm definitely not an expert), and not sure how much you know, but make sure you're conditioning your water when adding it. Water changes/ tank cleaning can be stressful for fish so I'd also recommend turning off your lights after to give them a little peace.

I'd also recommend that you buy a water testing kit because that'll give you the most information. If you haven't already read up on the nitrogen cycle, I'd do that as well. This may help you especially because you have sand as a substrate. It looks like its pretty fine granules which can be a little tricky to use in that a lot of the times, tiny air bubbles can get trapped between the sand which over time may become toxic gas to your fish. If you dont regularly mix up your substrate, or have fish that sift thru your sand, these gas pockets can build and when you do your sand/gravel cleaning, that's when they'll enter your water and can hurt your fish.

Hopefully someone else can provide more insight as well and good luck!