So I am struggling away to work out what type of co2 diffusing method I am going to use on my next tank.

Tank is around 900L, any thoughts or ideas please?

If you have a sump then an inline system connected to the return.  More chance of the CO2 dissolving in the water and the gas that is dissolved spreading to all the plants in the tank.  I used to have a diy system where water in the sump flowed through a container 1/2 filled with CO2 and the surface area inside the container dissolved the gas - worked well but that was only for a 125ltr tank.  The surface area needed would be too great for a 900ltr.

I'm dubious about any bubble or in tank system on any tank above 100ltr.  The more contact with the water more CO2 will dissolve, thus any system which lets the bubbles reach the surface is wasting gas.  The inline system means the gas is in contact with the water the length of the return plus the water flow cycling around the tank until it reaches the surface.

 

I used a similar system in my 750L, although the plants went amazing I went through a bottle (9kg) about every 5 weeks so was far from effective. I have considered doing another similar type thing if I can not find a pre made system