That's not always entirely true. I have had a few bettas and goldfish who have had very severe cases of fin and tail rot to the point where they had just about no finnage at all...They grew back perfectly normal and just as big and flowing as they used to be. Granted it does take a long time for them to grow back if it's pretty bad.
I have one betta right now who is recovering from a bout with fin/tail rot that was almost 3 months ago. Only a little less than half his tail has grown back right now, but you can see the new tissue forming on the ends.
Also, sometimes they'll grow back a different colour to what they were before...not always though.
Just keep the water clean and if you're treating with meds, make sure to follow the directions. If you have a hospital tank as well, that makes it all the more easier to med without destroying a cycled tank
Thanks CandyCat. I think my Kribs are looking much better than they did two weeks ago. I medicated with Melafix for about a week, then the Kribs appeared more stressed than when I started the medication. I did a 50% wc and added some carbon to the filter. Within a day, the Kribs looked remarkably better. Their fins all have shown considerable new growth and appear to get better each day. They really like the cleanest water possible.
Water parameters on Saturday were:
0 Ammo
0 Nitrite
5 Nitrate
7.0 Ph