There are a number of articles on this site about the gel food that I like to make and feed to my fish. I have been periodically admonished by several other hobbyists to not just feed the gel food, but to ensure that I provide some variety on a regular basis. While I certainly do not disagree with this advice, I have not been systematic in practicing what was preached.
Recently, I ran out of a particularly old and freezer-burned batch of gel food. I hadn’t had time to make a new batch. And, providentially, Vince McCarty (the GFSA advertising chairman in 1999) sent me a "care package" of assorted manufacturers’ goldfish food samples as a thank you for being a Report author. So I started feeding prepared food.
Two interesting observations. The first is that the oldest fish we had was a seven-year old Telescope Eye that passed away a few days ago. This fish had a rough life, being the "learning fish" upon which all care mistakes had been made. The fish experienced what I would guess was kidney failure about two years ago. The body bloated up with liquid like dropsy. But, alone in a 20L aquarium with good filtration and carefully fed on gel food, the fish was able to survive with no apparent degradation in its condition. But after two weeks on soaked commercial pellets, it gave up the fight.
Above: A seven-plus year old Telescope Eye who passed away in October 1999. She had been able to survive apparent kidney failure for 2 years on gel food. (Aside; note the headgrowth that had developed as the fish had gotten old. You often see this on Ryukins too ... there is a lot of mixing in these fishs' backgrounds).
The second, more telling observation is that I have some generally very healthy five year old Ranchu in a 55 gallon tank. One of the fish had shown a propensity to develop red, sore areas on the scales for most of the summer. I considered this a water quality problem and tried to change the water more frequently. I also added a cup of salt to the tank. This helped but the problem persisted at some low level with occasional flare-ups. However, without any water changes, the problem disappeared within a week when I started feeding the commercial food, and has not reappeared. While not definitive, this strongly suggests a deficiency in the previous almost exclusively gel food diet.
Above: Bravo showing a nasty red irritation on his scales. After persisting for months, it cleared quickly when a commercial pelleted food was fed.
So I am a convert. Even when I get around to making gel food again, I will still try to offer a variety of other foods (sinking pellets, frozen bloodworms, etc.) on a daily basis.