Ok, yes I was about to suggest the hickari goldfish foods lol
Use those as a staple every day diet.
Now to counter what I said before. every other day is good but what's better is everyday, and only feed a 'tiny' portion. like 1/3 of what you would normally give. say you normally give 10 pellets/flakes per fish, try to give just a little sprinkle in the morning.
This leaves the evening for another feeding time, but now you'll want to feed something different. The evening feeding time is when I usually feed the blanched veggies, the freeze-dried or frozen (or live food if I have it)
Also as a tip it's best to let all your fish fast one day of the week. that means no feeding at all for that day to let them clean out their digestive tract. Remember to also feed green peas 1-2 a week (in the evening or w/e)
This is how I would feed your tank:
One day- Fasting. NO food given
Second day- morning: Hickari goldfish pellet Evening: raw sweet pepper slice for pleco(leave the slice overnight), 2 leaves of blanched spinach for goldfish.
Third day- morning: Hickari pellet Evening: 2x Hickari Algae disks for Pleco (throw these in after lights out. plecos are nocturnal and the goldfish will not steal the food this way), 6-7 frozen green peas for goldfish (soak them in a bit of tank water to un-freeze them and then squeeze them to remove the shell. feed the mushy part)
Fourth day- morning: Hickari pellet Evening: large blanched romaine lettuce leaf for pleco(again you cn choose to toos this in for the night, after lights out, but it's up to you and whether the goldfish try to steal and eat all of his food), Frozen/or/ Freeze-dried Bloodworms for goldfish.
Fifth day - morning: Hickari pellet Evening: 2x Hickari algae disks for pleco, 6-7 green peas for goldfish.
Sixth day - morning: Hickari pellet Evening: blanched cucumber/zucchini slice for pleco, Omega One Freeze-dried Tubifex cube (one) for goldfish.
Seventh day - morning: Hickari pellet Evening: blanched broccoli for pleco/goldfish, and feed live food if you have any (ie. live blackworms or brine shrimp...NO TUBIFEX) Feed live food sparingly. for weeks where you don't have live food, it doesn't really matter. once a few months should keep them happy.
That's just an example and you can rearrange it if you want...
There are stocking issues in your tank, if it is indeed a 20 gallon...I can discuss those If you feel like it