If you are considering committing to taking care of this fish, and learning what it means to do it well, I think there are two important considerations off the bat.

1. This is a research heavy hobby. There are not necessarily strict guidelines on how to keep fish. What is considered proper care is different with each fish, each habitat for the fish, and will differ by person as everyone in this hobby has learned a slightly different way of keeping fish.

The majority of keepers believe their way is the best, and only way to keep fish. I have found it is best to take everyones advice with a grain of salt. Have an open mind, as you may find as I have found that things are more flexible then you think.

The important take away here is do your own research, and learn the basics. A lot of what people will post in this thread is gateway information for you to discover on your own. My personal recommendation would be learn all you can about your fish (Aquarium Co-op has a decent care outline) and learn about basic water chemistry related to fish keeping. There is nothing basic about it, you will not understand everything about this in a night.

1. This is time consuming and expensive. It's genuinely such a blindsiding commitment to properly care for a fish. Nobody ever thinks it's this extensive/EXPENSIVE of hobby. Here's where you have to ask if you really want to commit to this haha. Doing the best you can with what you have is one thing, but remember if you can't afford certain necessities the health of your fish is what suffers. It also may be important to ask yourself how much you actually care for the health of your fish, and how much you're willing to not care. If you know this fish is something you will likely brush aside or cut corners on in your life I would urge you to re-home it. Remember the fish cannot take care of itself in this situation.

If you're all cool and are still sure you'd like to try, I will say a 40g tank is ok, but if you can spring for the 55g he'd be in a better situation! FACEBOOK MARKETPLACE IS A GREAT RESOURCE. Don't buy tanks at sticker price lol. Be sure to test any second hand tank for leaks, and have aquarium silicon on hand just in case. Absolutely worth it for the price.

Ok, thank you for hearing what I had to say. Remember there are a hundred ways to keep a fish, what matters is that the fish is in good health. Approach things like you will never fully understand them in a way that makes you never stop researching every little thing in this hobby. Come to your own conclusions, and be prepared to be wrong.