So apparently there’s this idea that “flow” in an aquarium is important.  This seems pretty well established in salt-water reef tanks where anemones need to have some wave action to move food around over them but the benefit to freshwater setups is much less clear.  Good water movement in the general case is probably a good thing and helps avoid (possibly mythic) dead-spots in the water column where nutrients / CO2 don’t go very successfully?!? (doesn’t actually make a lot of sense to me, but there you go)

Lots of good choices in powerheads to push water around, but options are considerably restricted for an aquarium as small as 40 L.  Mostly this is because the powerheads are designed to push a lot of water, possibly with more than one in a given tank, so getting something that works well without blasting the inhabitants all over the place is a challenge.  Some potential options:

Of these, the Ecotech VorTech design is unique because the propeller that sits in the “wet side” mount adheres magnetically and spins by coupling magnetically to the “dry side” component that has all the electrical pieces.  That means no submerged electrical components, which is a good thing in the general case, but also means there is no need for an electrical wire to have to work up through the top of the tank.  Since the top of the Oase is not open, this is a big plus.  The downside to the VorTech is the cost… these puppies are expensive, running to £250+.  The conventional wisdom on these is to pick them up second hand on Ebay, so that’s what I did.  It was still expensive, but it did take some of the sting out.

The VorTech has lots of great programmable modes, including constant on, short pulse mode, long pulse mode (they call it ‘gyre mode’), and modes that simulate a lagoon (gentle), a reef (chaotic) and a tidal swell (both gentle and chaotic periods).  There are 12 speeds available.  I wound up going for gyre mode, where the powerhead alternates running at the lowest setting for 2 minutes, with running at the second lowest setting for 2 minutes.   That pushes lots of water around, provides some variety, and doesn’t go too crazy smashing things into other things.

Interestingly, the behaviour of the fish changed quite dramatically with the VorTech installed.  Previously the tetras would sort of lazily occupy the upper levels of the tank and the barbs would lazily occupy the middle levels of the tank.  With the flow on, both tetras and barbs spend a lot of time at the lower levels, surfing on the current over the bottom.  I suppose they enjoy that?  They do occasionally pooch up to the middle levels, but don’t really do the upper levels anymore, except at feeding time.  Since there’s no external filter on the Oase, I don’t have to worry the VorTech will push the food into the filter, so the flakes sail around crazily and the fish go after those vigorously.

Was it worth it?  Hard to say.  The dynamics in the tank are certainly very different with the circulation.  I suppose it works out to a plus overall.

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