goldfish, stress and tank sizes

Question
I have two gold fish (4 yr old is about a ft. long, 2 year old is about half that size) and a 4 in. catfish all in a 25 gallon tank.  My question is, why does my 4 yr old's fin flop over?  Is that an indicator of stress?  Gus only has it up when food is around, otherwise it's not just folded, it flops.  Should I get a bigger tank?  The only decoration is two live plants and a flat stone resting on three smaller ones, to give the catfish a place to hide.  none of my fish have any noticeable disease.  I just cleaned the tank and it's still cycling.  Is there anything I need to do for older fish other than the usual water change?  Can Sterile Gauze pads be used as a sack for the filter medium? (I buy bulk charcoal and ceramic and pillow fiber) How does a person tell what sex a fish is?

Answer
Alli,
you could use a bigger tank, the flopped fin can be a sign of water condition problems.
you should be doing 10% changes 2-3 times a week with fish that size.
yes you can use the gauze pads but a nylon media sack would be much better and about as cheap.
With gold fish sometimes you can see the females fatten up with eggs, or actually lay them when conditions are met.
Thats about the easiest way to tell.
If you have further questions,just ask.