You need a test kit to be able to check water parameters, this is essential equipment for fish keeping. The liquid test kits like the API freshwater master test kit that most of us use are more accurate than dip strips, and better value in the long term since you get more tests out of them than dip strips as well, along with the accuracy being very important.
You need to be able to test for ammonia, nitrites and nitrates when keeping fish, especially when cycling a tank or there's an illness going on.
A good video about the nitrogen cycle will help you understand why we test water quality, and how important filtration is:
Does the tank have a filter?
When you clean and change the water and clean the tank, what does that involve? What process do you follow?
Also what size tank is he in now? It looks like a 1-2 gallon vase type thing, maybe on a kitchen counter or something? Looks quite bright, open and exposed, and if it's in a high traffic area like on a kitchen counter, and exposed on all sides, he's likely to spook at every changing shadow and find it very scary and stressful. Fish are instinctively wary about predators, and a looming shadow usually means predator in the wild. We know that they're alone, but they don't know that, which is why we do things like add a background to a tank, and floating plants on the surface, it helps fish feel safer and less stressed since it replicates their natural environment and means they feel they can hide away from predators.
