Does anybody have any ideas how I can clean the tank thoroughly to remove
everything bad left in the tank? I want to use the tank (30 gal) to hold
my
baby convicts. Which brings me to part two of the question.

Lots of hot water, nothing else, nothing more... many rinses, and lots of
hot water... ohyeah, elbow grease... you'll need some of that....

The fry are about 2 weeks old now and I want to get them into their own
tank - What should go into their tank rock/gravel/filter/etc wise?

for a purely fry tank, heater and filter with something over the filter
inlet pipe or your fry will be sucked up the tube.... no rocks, makes it
easier to clean, no gravel same reason.... you basically want a bare bottom
tank so it doesn't get polluted...