Best filter & set up help needed

I have a 30 gallon tank that is home to my last fan tailed gold fish & a Koi.
The goldfish was part of a group of 15 that I acquired about 2yrs ago. They are gone to the heavens or to a friend. There was one more that was with him that died earlier this summer for unknown reasons. We then bought a Koi play mate for him.

Now, I have an Aquaclear 500 packed with a sponged at the bottom & loaded with 2 bags of bio max & a small sheet of floss in between.
There is also an Aqueon filter that is just using floss instead of the cartridge.
Last water test revealed that water quality was perfect. I do weekly 50% water changes. I do clean the gravel at the same time, even though there doesn't seem to be much buried unlike my 130 gallon tank.
But, there is a catch. The tank has a kind of musty smell to it.

Any ideas on how to get rid of the smell? Is it a filter issue or something else?
There are no other fish in the tank as I had to remove the Oto cats due to the Koi biting the head off of one. The rest were moved to the 130 gallon.

I do want to replace the Aqueon with a Bio wheel filter that I have sitting around but is missing the bio wheel itself.

Any ideas or thoughts?

Sandman - I have a terrible time answering your questions, because you already know the answers.I don't want to look unfriendly or unhelpful, but there is no answer to what you asked, either here or when you asked similar things on the old CAC.
A 30 or 36 inch tank holding a fish that should be 18 inches long within three years, along with a fish that should be a foot long and portly... forget filters. You need a 120 gallon tank.
And since an adult koi should reach 15 inches in two years and 31.5 (the average) within ten years, give up on small tanks and technological solutions. You've painted yourself, and your fish, into a corner.
Barring that, you need daily 50% water changes, or the smell will build. However, with such water changes, your fish will grow normally and soon be unable to turn around in the tank, let alone swim.
There is no cheap technological answer to your question - it's just the wrong question. You have fish that are too big, and tanks that are too small, and the only solution is a much larger tank with a couple of really big filters on it. Otherwise, you can drill the tank and build a continuous flow system, with 24 hour constant water changes. That's the only aquarium technologicasl solution, and it will cost.
If you have space, you can look at the thread Steve has on the pond he's building indoors.

I bet you wrack your brain when you see a thread from me. LOL
In regards to the 30 gallon, it is clean and clear. Just has an odor. I get the water tested about once a month at Big Al's. Water changes are done each Thursday or Friday at 50%.

My fiance suggested that we find the Koi a home & put the fantail in the 130 gallon with the 2 plecos, 3 ID sharks & a bunch of Oto cats.
That tank is amazingly clean & smell free. I do still have snails in there & they appear in my canister but not sure from where. Might have to take the hoses off one day & clean them out.

I added more of those pot scrubbers to the filter tub as well.

sounds like your fiancé had the right idea. 30 gallon tanks are too small for anything that gets larger than 4 inches at the best of times.

Sandman.. are you aware of the size potential of ID sharks? kept in suitable tanks, and I mean thousands of gallons and 12-15 feet long, these animals can reach 4ft long and weigh 80+ pounds. They are a schooling cat and do best in groups of 5 or more. People that keep them in home tanks, even tanks considered large by average hobby standards, are putting these fish to early deaths as they become stunted and die prematurely from internal organ failure. Same is true for Red Tail cats, Pacu, Peacock bass, Emperor cichlids. Your 130g will eventually become another graveyard for fish that are better suited for large ponds or public aquariums.

The one ID shark we have now, we got about a year and a half or so now.
I know all about their potential size.
He was rescued from a pet shop and was already kept in a 30 gallon tank before he was dropped off at the pet shop.

Have you ever considered completely changing your 130 stock? would make a great cichlid tank. Easier to care for, fish would do much better and you would have far less issues of filtration and overstock of too large of fish. You seem to be a cat/pleco fan, which is fine, but you simply dont have the room for these fish.

To be honest, the 130 gallon is at it's best ever. No smell, no water quality issues. All fish are happy.

Even the Oto cats are gobbling away at the shrimp pellets now.

On the topic of filtration, the pump it is using is a 330GPH that fills the 10 gallon tub filled with pot scrubbers, bio balls and floss.
I have a 600+ gph pump laying around, but am not sure how full it would make the tub. I am adding a 3rd output to it today to help lower the levels in it.

For that size of tank and with that stock, what size would be appropriate?

As for changing stock, the big ID shark & big pleco are both rescues, so replacing them is out of the question. Plus, around here, I doubt they would find a home.