Dark film inside dosing tubes

[reefer1]

Hi,
I just noticed a dark film developing inside the tubes used as part of dosing system. I have a bubble magus and I'm dosing alkalinity, calcium, and magnesium. The tubes are in the cabinet and are not exposed to light.
Any ideas on what is going on?
Is it okay for the ends of the tubes to be submerged under the sump water line?
Anyway, I plan on replacing the tubes tomorrow. Just want to know if there is anything to do to avoid this in the future.
Thanks!

 

[Randy Holmes-Farley]

The ends should optimally be above the water line to prevent precipitation of calcium carbonate, magnesium carbonate, or magnesium hydroxide from clogging them. So let them drip into the water.

All the tubes have gotten dark inside, or just one? Which one?

What type of tubing?

 

[reefer1]

The tubing is the tubing that came with the dosing unit. It was supposed to to be flexible but it didn't stretch too well and was hard to get over the connectors. All three dosing lines show the residue. I plan to replace using the flexible airline tubing from TOM's aquatics.

 

[Randy Holmes-Farley]

I don't know what it might be, but I'd guess it to be a property of the tubing itself and not the fluids you are using. I wouldn't worry about it.

 

[jcdeng]

I have the same thing, but only on my Mg line. Alk and Cal are clear.

 

[reefer1]

I just noticed (while changing out my hoses) my magnesium dosing fluid container has a dark cloudy sediment on the bottom.
I mix magnesium using BrightWell Aquatics Magnesium-P powder. 1/2-teaspoon per 150mL RO/DI water concentraion.
Is there a problem with the concentration or source? Anybody know what causes the dark cloudy sentiment?
Thanks!