Been reading these post with much interest....here is my take on the matter...............
I went to the pet store where I got the babies from and bought some sea salt.
When you say "sea salt" do you mean salt intended for marine/reef tanks?? If so then this is a needless expense.
Regular aquarium salt will do the job just as good, but at a much cheaper cost.
Please let us know the brand of "salt" you have.
I put the salt in the water as directed
Again, if it was sea salts, this is not good!! Directions for sea salt go by specific gravity and therefore you probably added to much!!
Usually, when I look in the tank, the fish have a reddish string-thing coming out of their underside (poop I'm assuming).
This is a good sign!! This is what your fishes feces should look like.
The weird thing is, the big one hasn't had that for the last 2 days. Instead, it has a faint white, hmm.. not sure what it is. It looks like a thread, but smaller, like a fuzz thread for lack of better words.
This is not good!!! This is a possible indication of an intestinal bacterial infection brought on by a case of constipation!! Constipation, if left unchecked can lead to intestinal problems!!
Try soaking the flake food in a small amount of aquarium water prior to feeding it to the fish. This will stop the flake from swelling in the fishes intestinal tract and causing a blockage.
The man at the pet store told me that it is probably in deed the fish's swimbladder, but he said it would most likely correct itself in a couple of days.
This is true if it was a simple case of constipation!!
He (fish shop guy) recommended that I do a 50% water change
This is also poor advice!!! NOT because your tank is not cycled yet, but because baby fish cannot handle this amount of a water change!!!
The man at the fish shop should NEVER have sold you that young a fish without having an established tank ready for them!! Young fish can be very delicate and should not be used for cycling a tank!!
it sounds to me like a swimbladder infection mabey you should get him out of the tank with the others and start him on some Maracyn -two
Listed are some of the causes of swim bladder disease.....
1. A virus. The virus attacks the epithelium of the sac and inflammation occurs which makes the epithelium too thick for gases to diffuse across. Thus the fish is stuck at a certain buoyancy because gases have nowhere to go. This may be more of a factor in non-goldfish species.
2. A bacterium. There is little evidence to support this, but it's widely known that bacterial infections can cause the same kind of thickening of the swim bladder epithelium as viruses.
3. Anatomy. Globoid-shaped fish like ornamental goldfish are predisposed to problems with the swim bladder because their guts are all squashed up in their abdomen. This arrangement predisposes to food impactions, which in turn clog up the pneumocystic duct.
4. Diet. Feeding dry foods which tend to take on water like a sponge and expand in the fish predispose to food impactions. See # 2 above.
If it was a swim bladder problem, IMO it was not caused by a virus, but by a constipation problem that affected the swim bladder. If this was indeed the case, than it is NOT contaigous and medication would not help!!
Also, most medications are dosed for young adult or adult fish and NOT babies!!! They (medications) should be used VERY cautiously when dosing babies!!!
If you are worried you have over fed them, go one full day without feeding them anything
Good advice!!
Hope this helps..........
CM