[gnatster]

In effect you would be putting pumps in series...

From Aquatic Systems Engineering by P.R Escobal

Suppose pump 1 has an input pressure of of 5 PSI and devolop a relative pressure of 8 PSI. It therefore inputs water at 13 PSI. But if this water is fed into the intake of pump 2, the pressure will be raised by the realitive pressure of pump 2, e.g.. if pump 2 has a relative pressure equal to pump 1, namely 8 PSI, then the new pressure at the output of pump 2 will be 5+8+8=29 PSI. Elevated pressures are often desired for the purpose of raising water to heigher elevations. However, one must keep in mind the pump seals, basicaly the "O" rings can only resist an upper pressure limit and the seals may buckle if three or four pumps are connected in series.

In your example of placing 2 cannisters in series keep in mind that the second one can only pump as much as the first can push out. So if you place a 300L/h cannister inline before a 500L/h one the second one can only flow as much as the first. It will however raise the pressure at the return being the intake is at a greater pressure then if not in series. I would think this may tax the seals of cannister # 2. Placing the higher flow one first may cause issues as well. The restriction of the lower rated one may tax the motor and place undue pressure on the return line out of the first. What may be a better approach is to have seperate drain lines to each cannister and one return connected with a "Y". Using a "T" as oppsed to a "Y" may cause issue with the more powerful of the 2 pushing against the other where a "Y" will allow them to blend.

 

[gnatster]

Why not just buy a recirculation pump and pump the flow up through a diy canister packed with filter floss and bio material and then back down through a diy co2 reactor, then return to tank?

Thats an alternative, as would using a sump. All would attain the same goals, only using different methods. Also not everyone is at the same comfort level when it comes to DIY projects.