Water changes definitely aren't stressful...and even less chance of it when they are used
I pull very large
water changes out of habit...on the order of 80-90%...and the fish,lying on their sides,just look at me waiting for the new fresh water. A side efffect is everyone's ready to eat as soon as the tank's filled,so if stressed they don't know it.
I'm not suggesting that for you at the moment. You need to pull just enough water changes to get the ammonia/nitrite/nitrates down until the filtration takes over...when testing starts showing absolute 0 on ammonia/nitrite,then you can think about the larger changes.
The problem is,and always will be,the amount of filtration you have versus the stocking on that tank.His use of salt for stress relief is going to end up being a constant band-aid to this it appears (if unwilling to consider reducing the number of Os)...so filtration is going to have
to be increased to ever get a handle on it,and large frequent water changes will still be the norm just to keep nitrates down.
Far as the feeder Goldfish (or any other feeder fish) goes,they are an extra biological load on the filtration the minute they hit the tank...doesn't matter if eaten in seconds...so that's another negative against them at this point in time. To me,personal opinion,it's more a "thrill of the chase" thing on the feeders than a question of nutrition in his case...so he can try shrimp,crayfish,nightcrawlers,and insects (Os love them all) instead.
If I can get info from all if you, I will do while he is at work at night .
If this is "sneak maintenance",then I strongly advise against it.If with his knowledge,then go for it.
"Sneak maintenance" will definitely help the Os...but teaches BF absolutely nothing. Bluntly,when they do better he'll think it's because of his
way,not the tried and true way suggested,and you will either have to confess (which may lead to a disagreement) or bite your lip and eventually come to resent all the work you're putting in for his
enjoyment (which can lead to a disagreement).Better to just tell him you are going to do it..."Let's see how it goes"...and get it over with.