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The poor real-world flow rate of these filters is the main reason why I never purchased one and quite often people complain about the flow rate. The advertised filter flow rates are of the pump and do not account for any filter media, hose runs or attachments. The main reason people use these is for the built-in heater and oase investing heavily into marketing to push their brand and filters - hence why nearly every man and his dog on youtube are reviewing and using them.
Have you changed any the media over? you will be better to keep the filter setup as per the factory....I think anything like eheim substrat pro would be more restrictive than sponges.
Yeah - to be fair every other manufacturer advertises flow rate in the same way I think. If the graph above does indicate flow based on head height, then a 1.2m height (which is probably not far from typical) halves the flow rate!
I think the main benefit of this unit is the easily removable pre-filter, not just the integral heater. That is the main reason I wanted it.
I tried removing the heater last night (as I'm not currently using it) and that increased the flow rate slightly also. I think the flow is now close to what it was with my Eheim 2028. I don't think the Substrat Pro is likely less flow restrictive than the sponge to be fair, though the standard Subatrat might be, the round shape of the Pro allows decent gaps between. The Substrat Pro is certainly much more flow restrictive than the K1 plastic media that Oase include in the box though, which I imagine you need to use for optimum flow rates.
The only difference between the Eheim, I had set up before, and the Oase in terms of filter media loading is the pre-filter sponge - everything else is almost like for like, so I guess that soaks up the 200lph power rating difference between the two. That said the open area in the Eheim filter basket bases (number of holes) is much greater that the Oase.
All in all I do like the Oase, I think it has been very well engineered overall, and the design has been well thought through. The flow isn't 'poor', I just had higher expectations of the flow rate given its 1250lph rating, so I just wish they'd strapped a bigger pump on it!