Welcome to AA!
I would suggest against NOT starting the tank from new .... you really mess up the biofilter doing that, & likely lead to more cloudy water & more stress for the fish.
I would also recommend against the flocculant water clarifiers. They work by clumping up particles so your filter can pick it up, but also gum up gills as well. Besides, they don't work very well or for long....
First thing to do is to find out why you are having the cloudy water. Being without light, I would suspect that this is not an algae outbreak. This is more likely a bacterial bloom. Bacterial blooms happens when there is ammonia or other wastes that are not handled by your biofilter. This may well indicate that you are exceeding the capacity of the tank.
I suspect that the bloom started because you changed out the filter & removed too much of the biofilter & the tank is going through a mini-cycle. It would be useful to test your water (ammonia, nitrites, nitrates). Replacing the substrate & decorations is counter-productive at this point, since a lot of your filter bacteria will be on the surfaces, and removing them will jsut make the problem worse.
My suggestion at this point is:
1. Test your water.
2. Do water changes, maybe 25% daily for the next while to reduce waste levels. <Depending on your water parameters, this will need to be adjusted.> Do gravel vacs to clean out detritus in the substrate with each water change.
3. If you wish to change out the substrate or decor, or filter material, do it slowly. I suspect that the tank is over stocked in terms of bioload, and you need to keep the biofilter un-disturbed. Generally, you don't need to replace filter pads unless it is faling apart ... most of us just rinse in tank water & reuse. When you change the pads, change no more than 1/2 the material (less is even better) every 3-4 weeks. Ditto for the substrate & decor ... small amounts every few weeks, this gives time for the filter bacteria to colonize the new surfaces & prevent mini-cycles.
4. You may consider getting a bigger tank .... if your nitrates are sky high or you are having problem keeping ammonia/nitrites at zero. Ideally, 4 golds & a pleco will need to be in something like 50-60 gal
tank when full grown. <In smaller setups, you will need to do a LOT of water chagnes to keep everyone healthy.>
5. If you have a canister filter (or can borrow one) you can clear up the water with a polishing pad/filter. Takes an hour or 2, but you will need to repeat every few days until you solve the underlying cause of the cloudiness. Having crystal clear water is only for our sake ..... the fish cares more about the water parameters, so I would work more on keeping the water clean & less on the cloudiness.