A tank of 2x1.5x1.5 has a 100% volumne of 127liters, and around 102 liters at 80%. We usually take the 80% range as the tank contains gravel/rocks and basically mass of water displaced by what we put in.

The ideal flow of the filter is said to be 3x-5x of what your tank volume is, but if you can go higher as better flow rate is better. However don't compromise your plants and its inhabitants (fishes find difficulty swimming, plants swaying too much). Just try to hit a maximum flow rate where the fishes and plants are ok with.

For the two taps, open the inlet from the tank to the filter to max. Do not adjust the inlet. Only adjust using the outlet, that is the point where the filter returns water back to your tank.

Any idea what is the rational why we should not adjust the inlet?

coz if u adjust the inlet, the motor will not be running at it's max and thus will shorten it's lifespan

The reason for not adjusting the inlet is that if you restrict the inflow, there may be a lapse (or water gap) because the impeller is driving out the water at a rate faster than the water is coming in.

You may experience airlock if you are lucky. Worse case the motor might burn out and you're left with a dead filter head.

-Calvin

Both my valve is opened at same mark, abt 3/4 of the valve.

Since both my valve is open at a same mark, water going in and coming out should probably flow at a same rate.

I tried opening the inlet to max and controlling the outlet. 2 hrs later the floor is wet, probably water intake is more than the water flowing out, then pressure caused the water to flow out of the filter.

Then I think there is something wrong with your filter. There should not be a problem where the inlet, being opened fully, will cause enough pressure to cause the filter to leak.

Check that you have the O-ring well secured in the filter.

Too many factors will cause the inlet and outlet rate to differ. As time passes the filter media will get clog from the dirt/mulm. The pipings and motor head themselves do decrease in efficiency.

If you see most cannister filters, the inlet will go through the connecting holes of the media tray in the filter, and the from the bottom go through all the media.

Then won't your filter most likely leak again in the future? Do make a check on your filter setup again.

-Calvin

Imho a 2 footer 600 l/ hr is good enough.