If the black scum is what I think it is, it can be dealt with but clean out is only the front part of the problem. It sounds like filementous bacteria. The good new is that I do not think the most common forms are fatal! Seriously it can be cleaned out if that is what it is. The trick is getting the bulk of the scum out. Then you do a Clean in Place -CIP.
I am assuming you have an organic scum such as filementous bacteria. That stuff can grow in a warm water disharge if it is a constant flow, it does need a food source but it makes the most of what is there. Try to figure out what best disolves the material and not the tank. try some of the material you extracted and see if it disolves in hot water (most orgainic scums will cut loose in HOT water. If that does not work, try it twice, agitation helps.
If that doesn't do it, then try something slightly acidic like very dilute muratic acid or even vinegar. If that does not do it try something a little caustic. Very hot water will take most organic goos out, you may wish to put some detergent in it to enhance the wetting power. Something such a liquid plumber can be used if other things fail. I once used it to clean out a water tank that some genious
decided to put koolaid in (of course it did not get cleaned righ away).
Assuming the detrgent does it, rinse several times, then acid rinse (very dilute muratic (hydrochloric), rinse again then use a very dilute caustic (Chlorox) and rinse again. The acid it to sterilize, the caustic is to neutralize the acid. The rinses are obvious. Keep the concentration very dilute.
If you have a dairy nearby ask them if they could get you a small amount of CIP chemical rinses (rinse, then food grade caustic, then food grade acid, then rinse it again anyway!). CIP means Clean in Place...I know someone would ask what that is.
Try not to use organic solvents. If you must, use something like alcohol not petroleum distilates. Not all organic solvents are bad it is just that the post clean up rinse must be very thorough and sometimes even a progressive rinsing like alcohol then hot or cold water--what I am saying is they are more hassel and have a higher hazard probablility.
Hopefully you can do this in place. Good luck.