Getting the right flow

[Essjay]

Yes it does, but the amount of water flow through the filter is not the critical factor. In fact some people claim that a slower water flow allows more time for the bacteria to remove ammonia and nitrite. As long as there enough bacteria - and they are on every surface in the tank and in the substrate as well as the filter - a slower flow rate is fine.
Your videos show harlequins and they do not like a strong flow.

 

[kevfiz]

ok but does a slower flow not decrease oxygen also?

 

[Essjay]

The Biopower 200 is rated for 100 to 200 litre tanks with a flow rate of 210 to 650 litres per hour. If you have it set to maximum flow rate, 650 litres per hour is far too much in a 70 litre tank. That's almost 10 times the tank volume per hour. The most you want is 5 times tank volume per hour, ie 350 litres per hour, and less than even that is fine for fish that prefer slower moving water. I would turn the flow rate down, not quite all the way by maybe to just a bit more minimum and nowhere near maximum.
(Eheim externals give the flow rate with media fitted; I assume they also quote flow rate for internals with the media in place)

And if you have it, replace the outlet tube and venturi with spray bar. That will spread the flow across a wider area.

 

[kevfiz]

the only way I am getting oxygen into the tank is on full power, if I turn it down slightly there is no oxygen getting in

 

[Essjay]

Oxygen gets in by dissolving from the air. Even with no filter in the tank, oxygen would dissolve. Filters increase the amount of oxygen that dissolves simply because they constantly move the water round the tank. The oxygen rich water at the surface is pushed down, and the oxygen depleted water from lower down gets pulled to the top where oxygen can dissolve.
As long as the filter moves the water round the tank, that's all that is needed. It is stressful for fish to spend 24/7 swimming against a current that is too strong.

I have a Biopower 240 with the spray bar in my 180 litre tank. That is rated for tanks up to 240 litres, and has a turnover of 280 to 750 litres per hour. Set at maximum flow, that gives 4 times tank volume per hour, less than half of your tank. I have never seen a fish gasping for oxygen.


Please turn the flow rate down. The reason your fish were swimming at the front of the tank was simply to get away from the filter flow as best they could.

 

[Byron]

the only way I am getting oxygen into the tank is on full power, if I turn it down slightly there is no oxygen getting in

The flow in both videos is too much for the fish species you have, and it will wear them down. Remember, fish are forced to deal with the current 24/7, and that is like you walking up a steep hill continually with no respite. The fish will soon give out.

Oxygen is important, but you don't need to be extreme over it. What I would suggest is either using a spray bar as essjay mentioned, but direct the holes into the end wall not down into the tank. Or turn the spigot you now have 180 degrees so it is aimed into the end wall behind the filter, again not down the tank. This dissipates the current. At the same time, ensure either is creating some surface disturbance at the filter, this will deal with the gas exchange (oxygen, CO2). Having this at night is also important as that is when CO2 builds up. It is simple to do this by adjusting the filter either way and ensuring it ripples the surface at that spot.

 

[Essjay]

The simplest way to reduce the flow is to turn the flow regulator knob towards the minimum setting. A tiny bit above minimum is perfect for a 70 litre tank.

 

[kevfiz]

Hey i dont know if its anything to do with turning down the pump but one of my beautiful blue guppies is turning a pale yellow. He is still swimming around and apart from colour change seems fine