I agree with most of what Behold said.
jonboy8465 said:
what length does it need to be? I will be using 1 1/2 pipe.
It's up to you! I'd look at somewhere between 30cm to 50cm but you could go longer or shorter if it makes it fit somewhere better. Obviously the longer it is then the longer length there is for the CO2 to diffuse so you'll get less bubbles going into the tank as the CO2 will be completely dissolved.
jonboy8465 said:
Will a fluval 105 be big enough to run it on a rena 450?
Is the Rena 450 a 450litre tank? With the fluval 105 you won't have great flow so you may find it hasn't got enough poke to really mix the water and CO2 up. I used my previous CO2 reactor with a 203 and it did a great job on a 180l tank. You could just use an external pump (like an Ehiem universal pump or similar) on a separate loop to your filter. Or you could plumb the reactor into your main filter pipework.
jonboy8465 said:
Is the reactor best fitted on the filter inlet or outlet?
Definitely the outlet as that is the side being pumped back into the tank and you want you CO2 enreiched water straight back with the plants.
jonboy8465 said:
Will the co2 affect the filter internals?
Not in my experience with a CO2 reactor after the filter. In fact many pump CO2 straight into their filter and don't have problems.
jonboy8465 said:
Can i put any filter media into the 105, if so what?
Sure but make sure it doesn't slow the flow any more if you do use this though. If you have another filter then I would either just put a few bits of biological media or don't bother putting any in to keep the flow as high as possible.
jonboy8465 said:
Does the reactor need anything inside to help break up the c02?
No it doesn't, but you can put some open void media to help break the flow up as Behold said.
jonboy8465 said:
Can i do away with the top tee and have the water enter straight in like Ed seeley has? surely this would stir up any collected c02 dissolving it back into the water column?
I prefer this design as I've used it before but either design works well. The tee design has the big advantage of making it easier to access the reactor to clean it. With the straight in design you have to unscrew the reducing bush that screws into the top so it's not quite as easy to access but it can be done without too much hassle. The tee design also mean you don't have to sort the filter pipes to enter the reactor vertically down so it's a bit easier to fit it in.
jonboy8465 said:
Am I correct in thinking :
the water enters the reactor at the top?
c02 enters at the bottom?
Actually I prefer to have both water AND CO2 entering at the top, already in the flow. To do this you need to add the CO2 in line just ahead of the reactor using a reducing tee. The bubbles of CO2 will get jetted down, already mixing with the water, and will swirl and diffuse in the slower velocity water of the reactor. Some gas will build up at the top but the water constantly jetting through it at the top will diffuse all that gas IME.