Coldwater community tank...

Plecos are not a good idea. They find the goldfish's slime to be quite tasty. if you google it you'll find some horror stories. hillstream loaches like a heavy current which would not make the goldies very happy. white cloud mountain minnows could work, unless the goldfish is fairly large (in which case he may eat them). dojo or weather loaches are a good match, but you need a larger tank (dojos do best in groups of at least three) and grow pretty long)
 
butterfly plecos are not PLECOS for your information, they are actually hillstream loaches. if you are going to use abbreviations, at least abbreviate a correct name for the creature you are talking about. and no, they do not belong in a tank with goldfish because they need heavy current, high oxygen, very clean water, and a large supply of algae to feed on.
 
1'x4'x1' is about 30 US gallons.

Leave well enough alone RTW, 30 gallons for one standard goldfish (shubunkin) is barely enough (shubunkins are ideally a pond fish)... and that filter isn't going to be sufficiant over the long term - the bigger they get the more waste (ammonia) they produce.
 
there is going to be a new external filter installed along side the existing one. So there is MORE than enough filtration now.

Anyone ever kept Rudds?

RTW
 
Personally I think you should have just left the tank as it was, one goldfish is plenty for a 30 gallon tank.

I'm just going to have to assume that you don't know how big koi get, because no-one who did know would put even one of them in a 30 gallon tank. I could be mistaken but I thought the average koi can get between 2 and 3 feet long, way too big for a 30 gallon. IMO they are strictly pond fish. Please return them unless you have plans to move them to a suitable home once they grow a bit.
 
You should have left it alone. No matter how much filtration there is, filtration does not remove nitrates.

It is bad enough that you even added fish at all, but you added koi. Koi are not suitable for aquariums because they grow huge. Koi grow 3 feet long will weigh well over 20 pounds. You are going to cause those fish to stunt. I hope you have a huge pond you plan on putting them in this spring.
 
I will agree, adding 3 koi to that tank is going to have a huge negative impact. You will be lucky if one survives due to their space needs, their secretions, and their wastes. No filtration in the world is good enough to support 3+ koi in a 30 gallon tank or however big it is. If you don't know of someone who has their own pond or have one of your own, I would suggest bringing them back. Those darling little things you found irresistable are going to be doomed if you leave them in this tank.
 
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