I'm going to venture that because of the algae, you could have a phosphate/nitrate problem. It's just being consumed fast enough that it might not show up on detection kits. This would also hinder coraline growth.
Another factor that could hinder coraline growth is the lighting. Apparently coraline does best under bluer/actinic lighting and not daylight bulbs. So you could try swapping bulbs to generate more actinic light.
How deep is your sand bed and how much flow do you have in the tank? Are there any dead spots where decaying material could be building up?
What type of skimmer is it? Do you have a sump/refugium?
You might want to try running phosban and activated carbon to remove additional organics from the water column. And 6-12 gallon WCs per month doesn't sound like much for a 75 gallon. IMHO, at least 25% would be more ideal at least so far as replenishing minerals and removing excess nutrients.
Another option to kill of the blue-green (slime) algae is dosing with Erythromycin (an anti-bacterial medication). Be cautioned that this is a more extreme approach and can result in the reduction of beneficial bacteria, as well as the release of chemicals from the algae into the water. Therefore you should run carbon if you do this approach. I had a massive slime algae outbreak in my own tank. After dosing with Erythromycin, it disappeared and never came back.
One last thing, are you testing magnesium? You didn't mention it in your list and its important to keep magnesium levels maintained. But you probably already knew that.
Joined: 03 Jan 2007 Location: Cape Coral, Florida, USA, Earth
Posted: 2008.02.20(Wed)8:34 Post subject: answers and more questions
In the reading I have done online over the last week or so, I think you have probably hit the mark pretty squarely. I probably should do somewhat bigger water changes _________________ I'll jump off that bridge when I get to it!
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