tjbuege;75918 said:
...or is the drain tubing big enough to handle the flow?
Aye, you do have to plan accordingly.
I use one made from 1/2-inch PVC plumbing on sensitive things like my Apisto fry tanks. That way, instead of drawing a large percentage of the tank and creating an almost instantaneous dilution delta, I add fresh water at one end while drawing off the mixed water from the other and freshen the tank a bit more gradually. Once I have the flow set up and going, I can also pull a little bit more water out while vacuuming without disrupting the process or having to stop the fill while I'm emptying the vacuum waste bucket (I know - buckets. So I'm Old School).
I looked up my original inspiration to find I'd mislabeled the system setup as "open" range when it's actually called a "circulating" range system. As illustrated on page 40 of Feroze N. Ghadially's "Advanced Aquarist Guide", published by The Pet Library, Ltd, of the Netherlands in 1969, it looks like this:
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where the lifting force for the tube connecting tank F's flow to tank A is simply an air pump. An air lift can usually supply a 2-3 gph flow, and that's enough to keep the inter-tank exchange to a decent level.
Using all of the tanks for seperate species tanks allows for the same individual spawning/rearing areas while taking advantage of the combined and inherently much more stable total volume in circulation. When I had the space, I've run it this way with a few double racks to more easily maintain breeding conditions across groups of fish that spawn under the same conditions.
Things like the Daphnia tanks I mentioned are another way to set up the downflow tanks. Ghadially mentions using the nutrient-rich flow to feed a high light, algae seeded tank to maintain Daphnia colonies. Now while I've seen folks do this with waste water before, I don't know of anyone trying to reliably filter algae from the flow within a loop like this. But I have seen some who run a Duckweed tank followed by a Hornwort tank to soak extra nutrients out of the water in the loop. And I've even seen one where the last stage was a trough instead of a tank, where the gravel-filled trough was used to grow emersed plants to serve as sort of mini-wetlands...