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I have been reading up on some of Darrels old posts about the importance of oxygenation of biomedia, and filtering in general.

It seems that the old school external filter (eheim style) that works both as bio and mechanical/syphon, is not necessarily the best way to go.

What I'm slowly realizing is the following:
Oxygenation is more important than volume of biomedia

I have an eheim 2180 loaded to the brim with both biomedia (3 full baskets of different sorts), blue filter pad and floss.

This is all according to the directions of eheim. The result however, is that the floss inside the filter, is completely dirty after 7 days. Also the flow, because of the massive amount of biomedia, does not get (I fear) sufficient amount of oxygen for its important processes. In the end, my water stays somewhat cloudy and has bits of debris suspended in the water column.

I do have a Maxpect Gyre 250, to circulate the water (5000g/h, at max), so there is plenty of circulation in the tank.

Apart from not overfeeding, having too big a bioload/stock and perhabs not having enough filter capacity (remains to be seen), I will try the following:

• Add prefilters to my intakes, so as to catch debris before it enters the external filter.

• Remove 1/3 of the biomedia to increase flow through the filter, and thereby increasing oxygenation

• Perhabs remove the white filter floss that works as mechanical filtration inside the filter?

Any thoughts?

I think you are about to draw similar conclusions to the ones I've drawn. I used to filter primarely with external filters, but the small tanks had internal filters, with blue foam and a small pump (4-8 watts).

The tanks with large external filters did not florish as well as the small ones, and when I had room for another large one (2.90 M * 80 cm footprint, that is over 9 feet * over 2.5 feet) it was build with a largish internal filter, blue foam and a 4500 l/hr pump.

The fish in this tank did much better then in the other tanks, and slowly the conclusions got their shape.

What we want in the filter is, basically, 2 things: removal of debris and ammonia (NH3) -> nitrate (NO3). The first is the mechanical function, the latter is the biological function. However, any debris cought in the filter wilol, ultimately, be broken down to its components. This uses oxygen, just as the biological function does.

So what an external filter primarily does, is take oxygen from your tank. It does give someting back, but the water coming from the filter is quite depleted of oxygen.

Obviously, it can be alomst free of oxygen in case of slow flow, dirty pump or rather rich in case of a fast flow, overly large pump which was recently cleaned.

When this oxygen poor water is returned to the tank at the surface, the water is able to improve dramatically - but where you would like to have the water add oxygen to the tank, you are working not to take oxygen from the tank.

An internal filter of the above discription does, basically, the same. It also collects debris, and it also has the needed biological function. BUT it allows oxygen to enter into the filter through its surface.

Further, in most cases the pump is not hindered by a dirty filter. A dirty Eheim can give less than 10 % of the flow it gave when clean, but the pump in the internal filter just pumps.

This can either result in a dry running pump - that is, a burned one - or a pump which lifts the water a bit. This lifting will reduce the flow, but in most cases just slightly (obviously, a 4 watt pump will be much more reduced by 10 cm/4 inches than a 40 watts pump).

When the filter is over 50 cm high, I still need to see the first pump get burned, but in 30 cm water, the pumps really are at risk.

From the above, I think you can answer your questions. Less stuff in the filter will imply less obstruction, thus more flow. It will also result in less bilological activity thus less oxygen use. BUT the bioological function is still needed, therefore this does not help much.