I don ot think the flipping is a choice - it is a problem.....
Flipping can come from a whole host of reasons - any of which can be present in conjunction with any other of them. Combinations - sometimes complicated ones - can cause the flipping - and to solve it, you may have to address ALL the problems. This is not necessarily easy to do. It is not a case of elimination, for, many times, the various problems are stacked - each causing the other......
Nitrates - as you have noticed can cause flipping.
Stagnant food or eggs in the gut can ferment and cause flipping.
Stagnant food or eggs in the gut can cause a bacterial infection and cause flipping.
I have had fish that respond (after about a week of LOW nitrates and continued low nitrates) to just lowering the nitrate concentrations in their tank.
I have had fish respond to fasting.
I have had fish respond to green food exclusively for a few weeks.
I have had fish respond to warmer water.
I have had fish respond to a salt dip purge.
I have had fish respond to an epsom salt grain - injesting one will purge a fish.
I have had fish respond to a male being placed in the tank - forcing an egg drop.
I have had fish respond to water changes, air pressure changes and temp changes, forcing an egg drop.
I have had fish respond to medicated food - killing bacterial infections that were releasing gas causing flipping.
I have had fish respond to any or all combinations of the above solutions - all or just a few!
I have also had fish that never responded to ANYTHING, and I eventually lost them.
I tend to be agressive when treating a flipping fish. I truly think the longer it goes on, the harder it is to solve or the more permanent it can become. I just recently have run the gamant of treatments for a large ranchu I have. I finally added in MetroMeds. Within 2 days, she is swimming much much better and more solidly upright. She floats normally - not end up or nose up or upside down. I hope that this can be a permanent cure.... time will tell. I have been futzing with her for nigh onto 6 months now - I solve it and it comes back. I add in another solution, have a modicum of success - and then she flips again.
It is a difficult problem to solve. Do not be afraid to dive in to solve it. It does need attention, though. It is not normal nor a copycat condition. It is a potentially life threatening condition. They are not suffering - but are not as they should be.....
In answer to the gel food - yes, as I understand it, you can put any fish product in the food. I do not, specifically. I use the seafood flakes from Goldfish Connection. They are a wonderful combination of various fish. Theya re easy to incorportate in gel. I use any goldfish or koi food I wish - floating or not, by simply soaking it in the water that will be used in the food and squeezing out all the air.
I make a gel food that is exclusively greens, to be used for all my fish to give them the roughage they need. I use only fresh greens..... Peas (you can use frozen ones if you do not have a garden), Broccoli and spinach make up the majority of the green gel. I add in dandylion greens (grown pesticide free in my vegetable garden), various lettuces and green beans are also common additives. This gel food is fed anytime I want a fish to have only greens for a while.
Even my gel food that uses protein uses at least 1/2 of volume in greens. Spinach and peas being the majority in that. The food, itself, is green - even with the protien.