I was in the same situation. I got a used 55G with a sump, pump, bio balls and 2 power heads. I don't know if this is the best way, but this is what i did.
I soaked the bio balls in a pretty concentrated bleach water solution for a few hours, and while they soaked, i set the sump, overflow, pump and power heads up to a 10G aquarium. I filled this with a less concentrated bleach water solution and ran it for a day. (I added the bio balls after a while.) After a day of this i drained the bleach water and rinsed everything pretty well. (several 100% water changes.)
Next i filled the tank with a vinegar water solution and ran it for a day, then rinsed really good again. I did probably 10 100% water changes, and ran this for another 2 days doing some more 100% water changes every couple hours.
I was re-sealing the aquarium while all this was happening, thats why i took my time. Im sure there are easier and faster ways, but i know this worked well for me. everybody has been living healthy for about 2 months
I soaked the bio balls in a pretty concentrated bleach water solution for a few hours, and while they soaked, i set the sump, overflow, pump and power heads up to a 10G aquarium. I filled this with a less concentrated bleach water solution and ran it for a day. (I added the bio balls after a while.) After a day of this i drained the bleach water and rinsed everything pretty well. (several 100% water changes.)
Next i filled the tank with a vinegar water solution and ran it for a day, then rinsed really good again. I did probably 10 100% water changes, and ran this for another 2 days doing some more 100% water changes every couple hours.
I was re-sealing the aquarium while all this was happening, thats why i took my time. Im sure there are easier and faster ways, but i know this worked well for me. everybody has been living healthy for about 2 months