My first planted tank(36gal)

[daworldisblack]

CO2 tank size varies and its up to you how big you want to get - obviously 5LB is not enough for bigger tanks so most get 10LBs. I got a 20LB recently for my 25Gallon lol. So the size if the aquarium is one deciding factor ( determines how often you have to refill the tank ), the other factor is that the price difference to fill up a 20Lb, 10Lb and a 5Lb is not significant so its more economical to get a bigger tank and pay almost the same amount as for a smaller tank but have it last way longer. The last reason i can think of, is the size of the tank ( hight,width wise) and where they are planning on being kept. In view, under a cabinet etc.

[exv152]

So if I was to use CO2, for this size tank what would I be looking for? How big should the CO2 tank be and any particular brand?

Brand really doesn't matter, as long as it's certified correctly by transport Canada, and size all depends on how often you want to refill it. Like Rah said bigger tanks just last longer and you tend to use more co2 with bigger tanks. A ph/co2 controller will also save you more co2 because it will go off and come on only when needed. But it's not an absolute requirement. I would recommend a 10lbs cylinder, that should last you about a year or so.

[Peekay]

Those gourami are beautiful! 
Love your spiraling vals too.