Yes!
No, any tank will work.
when its empty
Depends on the connections.
A CO2 tank does not necessarily have to be upright. It depends if you want liquid CO2 or gaseous CO2. When I purchased a CO2 tank for a specific application, namely, CO2 snow cleaning, I had the option of getting a tank that either had a dipstick, that picked up liquid CO2 from the bottom of the tank, or one that had no dipstick so that it collected gas from the top of the tank. I selected a tank without the liquid pickup. So, my upright tank output gas. However, if I wanted liquid CO2, I would put my tank slightly upside down, and get liquid CO2. It is probably not recommended to run a commercial gas tank in a non-upright position. So, depending on what type of tank you have, and what type of output you want, the answer to your question literally is: depends. For farm and garden use, you are probably seeking gaseous output, and probably have a tank with no dipstick; based on these assumptions, yes, you will need to keep your tank upright.
There is usually no need to put co2 in a FISH tank.....Fish consume oxygen and give off carbon dioxide (CO2)...just like YOU do. The main reason for putting CO2 in a tank is if you have GOBS of plants and HARDLY ANY Fish....as it is PLANTS which need CO2 to grow. If you don't have enough fish to feed the plants enough CO2, then add a couple fish. This is hardly ever a Problem. HOWEVER:::: there is a Doodad that you can buy for your plant aquarium which will add more co2. But that's not too good for your fish, assuming that you have them...
No paintball gun will work without a CO2 tank or other propellant tank. without Co2 there is nothing to push the ball out of the barrel.
sure, gennerally there are 30lb co2 tanks and 50lb tanks that you can fill from, you will also need a fill station and instruction from a air smith on how to use it, so it is fairly expensive to own, but what a lot of people do it buy a fill station and then rent a co2 tank for the day from a welding shop or another place that will do it (scuba shops sometimes, propane stations, you just have to do a little searching) for a 50lb tank i think it was around $30-$40 so have everyone pitch in a few bucks and you are covered for the day
you need Co2 to fire any paintball gun, it is what physically moves the ball out of the barrel.
CO2, stored in the tank is highly pressurized. The paintball being shot is caused by it being pushed out by the rapidly expanding gas. CO2 also cycles and reloads the next paintball.
No co2 tanks are sold full.
"H" is not a size for CO2 tanks