Betta Fish: Filter ot Tank Advice

This is my little guy Ares!

I have him in a 19 litre plastic tank, there currently is no filter (the current was too strong for him) or heater (no heater because its summer here and the room temp water is warmer than the heater). Previously he was in a smaller glass tank with the same set up and items. I decided on this one because it had a light (and was affordable). He has been in this new tank for just over a week and he doesn't appear to be stressed out, he even seems more active with the light!

My questions are:

1. Is a plastic tank ok? The water seems constantly murky even after a water change. I never had this issue in the smaller glass tank even without the filter and with the same plants and gear in it. Is the plastic affecting the water or bio culture in the water?

2. Does anyone have any low flow filter suggestions? It would need to be an in water pump as it is a closed unit. My current pumps are far too strong even though one of them came with the tank.

3. Should I return the plastic tank and just get a light and set him back up in the smaller glass tank? As it seemed to not go murky insanely quickly (over weeks rather than a couple days) like this plastic one. I'd still like to get a suitable filter for it though.

4. I am currently conditioning a larger tank (64 litres) which has a filter that wouldnt be too strong for him and a heater. The plan was to get some other smaller fish species. I still need a light for this tank. Would this tank be too large for him? If not what other fish would be safe for him to share the tank with?

Any advice would be appreciated! Thanks.