Got an issue with the 130 gallon

Hey guys/gals, been awhile now.
I have an issue with my 130 gallon koi/goldfish tank. It's recently started to stink up the place even though I normally do 50% water changes every week or so. Tonight, I did maybe 80%ish and also cleaned out the fluval fx4 and cleaned the gravel. Still stinks.
Any ideas? Should I just remove most of the gravel? I also changed one of the media pads.

Hello Sandman1969,

Welcome back. Hope all is well. Koi/goldfish are flush feeders so there is a high turnover of organic matter. The huge quantities of food that you feed them could easily turn into swamp, as you know. So you have done most of the things but perhaps you may wish to reduce the amount that you are feeding them, as well as increase the water change (keeping the minimum one week). Definitely I would have suspected the Fluval but you have already taken care of this. Maybe use charcoal (which you may already have done). Gravel cleaning, you've already checked off that box.

As you have already done so, sequential cleaning as well as high and frequent changes. Maybe search for a decaying source, but I think you have done that. Perhaps check your water source. Is the water going in the same water that you drink? Do you fish look healthy? Aside from that, I can only guess.

I am going to try to incant the aquarium gods on this one:
@SBenson11 what do you think?
@GaryE any thoughts Master Jedi?

It is the same water that we use daily and they all look fine, as do the ID sharks in the tank next to it. That tank doesn't have a smell. I don't feed a lot of food to the koi/goldfish and use floating pellets. It's rare there's ever any left as the koi is a pig. I did do a full gravel clean last night and not much luck. Both tanks are in the basement so no real sunlight, so no algae buildup either. I am planning on removing most of the gravel as it's about 2 inches deep. That will be next weekend's plan. There's no dead fish or anything either as this tank has been set up here since last summer.

UPDATE:
Think it was due to the koi being "constipated" from eating gravel. He's currently in the second stage of an Epson salt bath and he's released several gravel.
Doing another water change to help get rid of the dirty/stinky water.