First off, what’s prompted you to get pressurised? You might not need to. That said pressured, once you've got all the kit, is far easier to work with IMHO. For arguments sake I'll assume you definitely want/need pressured CO2.
Adding CO2 shouldn't affect the fish. It can kill them should it get to high but that is easily avoided if you are careful, I've yet to lose any fish to high CO2, but it does happen and is something to be aware of but do let that put you off pressurised CO2
Adding CO2 will cause the pH in your tank to do down, normally by around 1 pH point, say from pH 7 to pH 6. This is often beneficial as plants like to have slightly acidic water. The CO2 can also cause your KH levels to go down, but this is easily sorted when you do a water change as this replaces the lost KH. Again I've never had a problem with KH, and you shouldn’t providing you do your regular water changes, which being a good fish keeper you will
Now, the need to add other things really depends on your light levels, if you have a high light tank then you are likely to need to add fertilisers to the water to give the plants enough nutrients to grow. These are easily added in liquid form, but I suggest for the time being you post your proposed tank stats and we'll have a look at that side of things later.
As for the kit you'll need, the most basic pressured CO2 kit has a regulator (most have a needle value built in, more on that later), a co2 cylinder (well of course!), a length of CO2 tubing (identical to airline tubing but designed not to degrade in the presence of CO2 which airline will apparently). Lastly you'll need some form of diffuser to get the CO2 into the tank water; there are several types but again more on that later.
Now the hardest part of pressured CO2 is finding somewhere to get your CO2 cylinder refilled, especially in the UK. I suggest you check with your LFS to find out if they can and which make of cylinder they will refill, as LFS in the UK will often only fill certain brands. I hear it’s easier in the USA with places refilling generic cylinders. This will probably determine which brand of CO2 gear you get.
The cheapest way of getting pressurised CO2 can be to get all the bits separately, however if you are not sure about it, a kit version might be easiest and for the sake of a few quid, might be worth it to know you’ve got the right things.
I'll go into more detail on the ‘ins and outs’ of pressured kits once you've read this thread and found out what you LFS will refill. Just so I know where about are you? UK or USA?
Sam
PS - good thing my boss didn't see me write all that!
