Just rebuilt my 60w system with a AWC068 controller and new 60w P.S. Went to cut some 3mm acrylic and at 80% power and slow feed rate, it took 3 passes. Then I noted it was only drawing 10ma on the tube current. Had been cutting ok before the conversion so what had I messed up on? After checking and rechecking wiring, P.S. ect, I noted the tube had a lot of film in it (I used water from a dehumidifier). Removed the tube and after looking at it outside, there was a lot on film and/or algae build up on the surface of the water tube. I had just recently seen a post from Walt in the UK about removing this film with a 5 to 10% HCL solution (muric acid ) flushed though the tube and even "shake" the solution back and forth in the tube.
I took the tube outside, hooked the water pump to it, added the acid to the water (NOT water to acid) and pumped 5 gal of solution though the tube. At first this did not help much, but had a slight improvement. Ran another 5 gal of acid solution though it, this time starting and stopping the pump a lot, as well as shaking the solution back and forth from end to end. Again some improvement but a long way to go.
My wife found out that DOVE dish washing soap is suppose to be good at removing molds, so I put 1/2 to 1 oz in the tube, pumped in clean water (pumped 5 gal clean water though the tube to flush any acid out first) and shook the soap solution back and forth. This was making some progress but still much to go. Repeated acid / soap cleaning ( with 5gal rinse of clean water between each) and was making more progress, approx 75% removed at this point. There where spots that just did not want to come out, like in both ends and a nasty area in the middle.
At this point I knew that another approach was going to be needed so I took some 12inch long "pipe cleaners" and inserted one into the glass nipple on the anode side, very carefully working it down into the tube, DO NOT LET IT KINK! or you may not be able to remove it! this DID remove a lot that remand, so over to the cathode side a got approx the same results. Now the ends where at approx 90% clean, but the middle still remained at 75%.
This is going to sound odd, but what I did next was to insert a 3plus ft of braided, nylon covered, stainless steel fishing leader into the nipple on each end of the tube. By very carefully "fishing" the wire though the nipple and down into the water chamber, I was able to rub the film and free it from the glass and then flush it out with the soap or acid. I was even able, with a lot of patience, to get the fishing leader to travel down the length of the tube to get at that pesky middle area.
So after 8-10 hours of cleaning and dish washer hands, i have a tube that is 99 plus % clean.
Now for the best part. After putting the tube back in, replacing ALL the hoses with new ones,cleaning the water sensor out, putting in fresh clean water, running the water though the system for two days to be sure of no air bubbles, I fired the laser up on the same exact settings as before and it just burnt its way though the acrylic like a knife though butter and the current went right up to 20ma.
So now, instead of replacing a $400 tube, I have one with a second lease on life.
So if you have a tube with not many hours on it, output lens is in good shape, BUT has a lot of film and junk in the water chamber, you may want it give this process a try.

Rick