Newbie advice about bio-orb

Hi

Bought a bio-orb a few weeks ago, just got a couple of newbie ish questions!

i've flicked thro here (after i bought it) and found people dont like the
bio-orbs, why is that? whats the difference between that and say a 35liter
normal tank?

have had problems with the nitrate/rite levels going nuts, think thats
because i overfed the fish too early and put in too many fish, at the moment
i've got 2 clouds minnows and one 3"ish chubby goldfish, i do 1/3rd water
changes every week and things seem to be ok again, i check the levels every
week and they seem to be more stable now, do you guys think i put too many
fish in too soon and that 3 fish will be ok for it?

also bought some pond weed type stuff which looks good, but the big goldfish
likes to hide behind it away from the minnows, he comes out every now and
then got a good swim then hides again, guess thats all normal?

also if theres any other advice you can give it would be much appreciated!