in those cases we will see staghorn algae in all tanks
It is not an algae that exists in all aquariums, and is always ready to make an appearance, like GSA. I never saw it at all in many years of keeping planted tanks, no matter the parameters, or how well or poorly they were set up.
Then I got a new plant. I saw algae on it, which I didn't find out until later was staghorn. So I dipped the plant. Then I put that plant in my most stable tank, in which all algae magically withered away; confident that anything the dip didn't kill would also suffer the same fate. I was wrong.
So no, we would not expect to see staghorn in all tanks.
if Mg and Ca is not the cause then what causes it?
Already posted my explanation of cause. And if staghorn isn't present, then the same conditions that support staghorn growth, if just a little more favorable, produce BBA instead. Which is much more common, present and ready to strike in all tanks.
Since staghorn can be completely absent, I think it may be possible to achieve total extinction
of it as well; so that it can never reappear unless reintroduced. I periodically push conditions into favorable areas to see how well it's working. And though I haven't yet achieved extinction, I'm making some progress - these pushes now generate more BBA than staghorn. Strange as it may sound, I prefer BBA!
it seems to spread no matter whatever changes i make
Heh. I know that feeling well.
i did notice that it seems to spread more when Ca is way less than Mg. am still testing it and i cannot be 100% certain if this relate to Mg and Ca.
But when Ca is way less than Mg, that's not good for the plants. And so this fulfills one of the conditions I've stated encourages staghorn growth. Eliminating this condition will only slow it if any of the other conditions still exist.
i am testing many algae in my tanks to find out what really causes them. i will soon report on my research.
All tests welcome. Shoot me a PM linking to your thread when you report, I'd love to see what you've found.
Ack, I typed PPS in my previous post. I really meant MCI. That's what I followed, and no help.