Hi,
In general more diffusers are better but on a 50Gal you ought to be able to get by with a single external diffuser. This size tank however eliminates the JBL system or even a DIY system from serious consideration if you are planning on growing the more exotic species or carpet plants. On the other hand if you just wanted to grow a tank full of Hygrophilas or Ludwigas then these systems might be fine. As usual it depends on what vision you have for your tank. How does the tank look in your minds eye? That will determine the type of equipment that will either be compulsory or that you can get away with.
An FE How To can be found in this Tutorial:
FE CO2
A more expensive option if you don't want to go the FE route is to join the BOC and then collect (or have delivered) the cylinders from any BOC distributor:
BOC CO2 Cylinder Sizes
There are also third party cylinder suppliers/refillers. Ask around at local pubs or welding supply shops.
Yes, external heaters and external diffusers do take their toll on flow rate. Ideally you'd like a flow "rating" of 500Gal per hour/2000LPH or more but this may not be practical due to size and available space. In that case get the biggest that is practical but be aware that depending on what you want to grow you may later have to supplement the flow with a powerhead. Remember that you can use multiple smaller filters as well as a single large one. Flow rates are additive so that two filters each rated at 1000LPH accomplish the same as a single 2000LPH rated filter.
It's been my experience that on larger tanks, combining high flow, while distributing that flow from the filter's effluent across the back of the tank using multiple spraybars generates a better circulation pattern than the single "point source" effluent of a lily pipe. The bars are not very attractive at all, so if this is important to you then avoid them and just use the lily pipe.
Cheers,
