Welcome to the forum! To start with, please read the pacman care sheet if you haven't already. Frog Forum - Pacman and Horned Frogs - Ceratophrys - Care and Breeding Frogs do require daily maintainence. Proper temps and humidity are Very important for a healthy frog.

1. The best substrate for them is coco fiber (not coco husk!). They are terrestrial and prefer to burrow. If you use a water/paper towel set up, it must be kept clean and fresh. Otherwise they are sitting in their urine all day. Some Pacs get very stressed out by such a set up. Long term, if they don't have anything to grip against on the bottom of the enclosure they often develop hip/leg problems.

2. It is most likely shed skin. If it is something coming from its urine then that is cause for concern. Give the frog a soak and watch when it urinates to see if it is clear.

3. Only males do the mating call. They can start around 5.5 months, although they reach full sexual maturity around 8-12 months. The females do make some small noises. A popping/clicking noise and an angry hissing that is their dismounting noise.

Like Pluke said, goldfish are not a good feeder. They carry parasites and have almost no nutritional value. Nightcrawlers are the best staple diet. You can cut off appropriately sized chunks for the frog. Generally rule is to feed items sized approximately the distance between the frog's eyes.

Let us know if you have any other questions. Post some pictures when you get a chance. We all love pictures here!