I would recommend looking up the type of fish you have and then figuring out from there the minimum tank size. You should be fine with a 10-20 gallon with a lot of fish. Based on the image it looks like a platy, molly or guppy and those only need around a 10-20 gallon for around 5-6+ (a school), but if it is a goldfish you truly have a problem. It varies by species of goldfish, but most, I think the bare minimum for one is around 30-50 gallons. If it is a guppy, you can keep a pair of two guppies in a smaller tank (5 gal. minimum, but even that is pushing it). For anything smaller than 10 gallons, with the questionable exception of two guppies, the only thing you can keep in your tank is either a betta or snails. Some might even argue shrimp, but I am not sure I agree with that for there are many reasons why most fish need bigger tanks. Among those reasons are that many fish produce too much waste for a filter to even be able to take all the bad stuff out (a filter can only do so much), so if stress didn't kill the fish first, the water quality would. Imagine being trapped in your house and never allowed to step foot outside or order outside supplies to be brought in. It's not manageable nor is it enjoyable. They need room to move around and swim their hearts out. Note though, erratic swimming is generally a sign of stress and the longer a fish is stressed, the more likely it is to catching something and dying. Or simply living a life of hell.
Also, if that doesn't have a built-in filter that we can't see, YOU NEED ONE. There is no species of anything that can be reasonably kept in a tank without one regardless of it's size. Even betta tanks. Those without filters need to do water changes daily, and that isn't fun for either party.
What is that tank size? 3 gallon?
As for the question you originally asked, I think that once you get a proper light, it should eliminate most of the glare from outside the tank, but I would recommend putting more décor in there as it will help 'keep the light in' (if you will) & counteract some degree of glare. Especially live plants. Live plants will make a massive difference in keeping a smaller tank healthy, longer. Also, imagine not having anywhere private to relax and de-stress. If fish don't feel safe, they won't sleep well (or at all) & will they live short, miserable lives.