You're tank is small, probably around 0.8 gallons or less. . . There's no heater for your tank, I would not recommend buying a heater for your tank unless you want to make fish soup.
Bettas come from tropical countries (Thailand, Malaysia, etc), they thrive in the temperature range of 76F ~ 82F. If you could, upgrade your tank to a 2 gallon. If not, I would not suggest buying a heater for your tiny tank even though it is greatly needed. There are other ways to heat your small tank but it's not recommended. .
Uhm, this might come out mean but in no way do I mean it to.
Look, a bowl is not a proper home for a Betta. That's a temporary environment. You can't heat it, you can't filter it, so... maybe not get another Betta?
Maybe instead of having lots of Bettas in poor conditions, you could have one in a thriving condition? With the money it would take to get another Betta you could get a nice 1.5 gallon starter kit which is a good home for him for the rest of his life.
Okay? Maybe you could reconsider this before you decide to take on more fish.
I would suggest saving your money to buy a better tank for your CURRENT betta. you have one in a tiny tank, and with no heater. so save your money for HIM, rather thenn buying another one and having 2 poorly housed bettas. this is merely my opinion, but i think it is a good one (:
I have the same tank for my QT tank. What I don't get is... you have room for two small tanks but not one larger one? You are planning to house 1.8g for two bettas but not 1.5g for one? I have the Tetra Water Wonders, comes with a filter and light and its not much bigger than the 0.8g Marina space wise. It's 1.5g. I have to agree with others that its better to work on the betta you have now rather than get another. It's tempting for sure, but think of Mr Jingles first. With limited misjudged, youre, ah limited lol
I used to use a .5 gallon and all he could do is turn 90^ then go about 2 inches then do it again. After joining the forum and despite the fact that i thought that is their primary habitat and that they liked it, i could tell hi didnt. Nothing by this, but i have since got a 2.5 gallon for my new betta (the other died) and its amazing to watch him actually occupy himself, such as brushing the plants, messing around in hes hiding spot, and freaking out for feeding time. I actually put a ping pong ball in the tank and he pushed it around for half a minute then scurried away when he popped it in the air and it made the water shiver. but ANYWAYS.... i dont understand how u dont have room for a bit bigger tank, since its hard to find a spot my 2.5 gallon doesnt fit in, i dont understand why you cant find one.