I've written you in the past on a couple of tanks that kind of fell in my lap. One was a 120 and the other a 140. Both tanks have done terrific on the trickle filter with live rock in the chamber, huge skimmers and lots of water changes. Maintenance is still 30 gallons bi-weekly, the only additives are PurpleUp daily and controlling the phosphate, which is zero as are the nitrates with calcium around 425 to 450.
My problem is with the 140, as I've battled a brown stringing hair algae like substance with bubbles in it, which appears to be some kind of cyanobacteria! It covers the sand and the rocks. I've removed all rock and brushed them clean twice, and I've changed 60 gallons of water out per week for the last 5 weeks, but it comes back, but a little slower each time. The R/O water I use tests out at 0 ppm for TDS. The other tank I have shows no sign of any of this. The tank is lit by two 96W PC lamps and a 175 14K MH. I've reduced the halide time to a couple of hours a day. All corals and animals are doing fantastic, which I'm glad of, but I'm ready for this brown algae nightmare to end. Thanks in advance.
Thanks for the letter and from what you describe, 'a brown stringy algae with bubbles in it,' it looks like you're describing a dinoflagellate alga. It is usually introduced on a new piece of live rock or coral animal and can easily become problematic if the bulk water is rich in organic material and can be difficult to cure because its free-swimming alga. But when it comes to brown 'stringy' algae, there can be two types, and to cure them you need to know how to tell them apart.
Probably the most common brown algae are the brown diatoms and these can usually be sheet or film-like in appearance, appear as tiny brown dots on the aquarium inside panels, or cover substrate surfaces where there is sufficient light. Silica/silicate is used in the structure of their cell wall and depending upon the amount of silicate in solution, their growth can be anything from minor to major labor-intensive cleaning tasks. Besides appearance, when heavily coating objects it can photosynthesize and produce oxygen bubbles, which in severe cases may raise pH to dangerously high levels (above 8.6). However, if your TDS is that low, I doubt silica/diatoms is the problem.
As for a dinoflagellate alga it does not need Silica/Silicate to expand its numbers, unfortunately it uses calcium carbonate to do that, and your calcium level is high enough to provide it what it needs. A small microscope would help identify which form of brown algae is in the aquarium, as the dinoflagellate alga cell has a flagellum, i.e., a tail (for propelling itself through water), the diatom does not. This free-swimming dinoflagellate alga can be reduced/eliminated with the use of ozone or a UV sterilizer. Also, flowing all aquarium water through a sponge filter, which should be cleaned twice a week, is also helpful. And if a 'quality' brand protein skimmer is not being used, it should be as this pest does best in high organic conditions.
Hope this helps,