01-08-2010, 04:05 PM
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Well, you're turning your tank over 5 1/3 times per hour with that filter. That falls within the manufacturer specifications, sure. In my opinion, you should be turning over your water much more than that, especially with such large, messy fish as you've got. I would recommend turning it over at least ten times per hour, meaning that another Emperor 400 at the other end of the tank would probably be a very good thing. Keep in mind that you've got a fairly large tank so water movement at the end opposite a power filter isn't going to be very good, which is why so much waste accumulates down there.
Now for the really bad news that you probably don't want to hear: your tank is far too small for the fish you've got. Koi can grow to three feet in length and are essentially unhousable in in-home tanks. They're much better suited for large outdoor ponds. Usually something on the order of 1000 gallons is suggested if you want to keep koi. I would recommend a larger tank for the barbs as well, as they grow to about a foot and and are active swimmers that need a lot of space. A 150g tank is probably about the bare minimum for tinfoil barbs, although larger than this would be ideal. Finally, the barbs are a tropical, warm-water species while the koi prefer lower temperatures, so housing these two types of fish together means that one of the groups is living outside of its comfort zone, dramatically increasing the chance for illness. If you have a pond for the koi, that would be much better. If you're set on keeping the tinfoils, you should think about upgrading to a larger tank as soon as you can.