I have two of those tanks (probably more in my room I haven't dragged to the fish room). I like them. They will require frequent cleaning and a small heater but they're great for one betta :)
I have them too and I like them. One of my boys can't live in anything bigger due to his huge tail. He has to have shallow small tanks and this works fine for that. He also can't handle a filter because of his fins. He hates water disturbance of any kind. This tank gets my vote for finicky fish. I socialize with them and they can see eachother. They don't just sit in small bowls. They are active and contrary to popular belief they can house a small cave and a couple of plants. just my opinion. Please don't attack me for voicing it everyone.
I keep 10gallons of clean water sealed and in a warm place if its temp is to low i drop a heater in it when i need it. it realy depends on your needs but I try to keep fast back up water ready in case i ever have to do a WC and the taps out(I live in CA and earthqauks happen) plus i have enuff water to transport all my boys if i need to (.5 gallon each) :)
There is no advantage to leaving it out. Yes, the chlorine will evaporate out, but you need to leave it for 2 weeks to get rid of chloramines and heavy metals will never go. Leaving it out allows stuff to fall in it and for the temperature to drop to room temp. Much safer just to use a good dechlorinator like Prime. :)
i read somewhere that you actually need a bubble thing (always forget what theyre called) for the chlorine to evaporate. i think it was one of the moderators... not sure who though. but they said that the chlorine wouldnt evaporate unless the water was oxygenated. or at least that it would speed up the process maybe. im not really sure but i figured id put that out there
I have these tanks, I now use them for hospital tanks only. Honestly, it was good for a start up but I upgraded quickly. The dimensions are kinda useless for Bettas, very tall and narrow. Once you add gravel and a plant, you're left with about 0.7g of water for the fish. Get a critter keeper instead, they will love the side to side space :)
Also, you don't have to leave water out for days. Just add a high quality conditioner like Prime or Stress Coat to your water, stir for a minute, leave for 2 minutes and add to the tank. Leaving it out may help with some stuff, but it does nothing for the metals in the water and it lets all the oxygen out.