Could i run a external filter which is can run at twice the size of the tank?
i have a eheim 2226 filter which has a main cap of 300l and my tank is only 150l. now i have heard that some people do have filters which are way to big for there tank to make the water quality immaculate. But is this recommended?
It would matter what type of fish you keep. For larger fish that create a lot of wastes (goldfish, large cichlids, large catfish) or ones that like a lot of current, overfiltration can be a good thing.
If, on the other hand you’re keeping fancy goldfish (egg-shaped body and double tails), bettas (long flowing fins), killifish (prefer slow water movement), fry, or other fish whose swimming might be impaired, then overfiltration isn’t such a good thing.
Generally, going up in size in a filter won’t cause any harm, as long as the size difference isn’t extreme, and your Eheim should have a lever to adjust the flow rate. You can also direct the water return toward a wall to disperse the force of the water when it returns. But, again, base this on the type of fish you keep in the tank and their swimming ability.