I thought I'd 'freshen up ' this post as I ran into the same problem today. My Python was starting to collect a lot of dark black mildew in the hose and I was becoming concerned about the refilling and health of my tank. This is what I did:
1) I got an extra one of my kid's sand pails (1/2 gallon or 2 liters). I put Chlorox in and filled with water, about 1:10, 1: 5 dilution
2) After doing my tank's water refill, reversed it to suction, emptied the hose and then suctioned up the Chlorox sol'n and then stopped the flow with the shutoff valve to keep the hose filled
3) left the sol'n in the hose about 15-20 mins. During this time, all the dark deposits literally disappeared
4) I then rinsed the pail out thoroughly with water, added dechlorinator, refilled the bucket
5) I turned the sink on, released the valve and suctioned out all the Chlorox and then flushed it with dechorinator sol'n.
Hose was spotless afterward. Make sure you rinse the hose well.
SH