We have recently got 3 rosy reds for a 5 gallon filtered coldwater tank.
1 week later one is lying on bottom of tank, often on its side,
poor balance when it tries to swim around. This is my 6 year old's
favourite of course! Water seems fine (ammonia/nitrite).

Any hope for it? I tried a 30% water change, then the next day
decided to move it to a small (1 G) tank (OK, box) in case it
was catching.. and at a loss what to do added 3 teasp sea salt.
Not much change. Is it constipated? What would one
do with a constipated minnow..
:(

I don't think the rosy red is constipated; I've not heard of that problem in fish! From your description, I don't know why your
fish is sick but it doesn't sound like there's a great chance for recovery. You can add aquarium salt (sea salt if the iodine isn't
too high; a tablespoon per few gallosn for treatment should suffice) and maybe some MelaFix by Aquarium Pharmaceuticals
which is all natural and won't hurt anything. Not knowing if the fish has parasites (extremely likely if this was a feeder fish
as most rosy reds are) or what kind or a bacterial problem or something else, I can't recommend which types of medications
to try. I don't know if the other fish have the same thing but if it's contagious, they've already got it (but may fight it off if
they are stronger). You checked the ammonia and nitrite which is very good. Did you check the pH too? Do you add water
additives with your water change like dechlorinator? I hope your fish make it! I just cleaned out my 153 gallon pond today and
found 11 rosy red minnows (only 3 male) but one girl was killed when I vacuumed the bottom (I thought I'd gotten all the
fish out), and I feel really bad for her.