I'm reasonably confident with my Ph reading, it fluctuates (colour) a little, but never enough to call it the next one along on the scale (which are 8.0 and 8.4), if I had to estimate I'd say it fluctuates between 8.15 and 8.22. I can also see it reads all ranges when I wash the vile out with tap water, I will try another kit though.
I understood that clams need a little nitrate (as opposed to just that they will filter out nitrate), as my nitrate was low I assumed a few more fish and hence more food being turned into nitrite and then nitrate (I haven't done any water changes yet).
I don't have any turbo snails yet (I have 25 on order now). I thought my brown algae stage would have come to an end now but it hasn't (in fact I missed a clean because my clean night fell on new years eve and was a bit surprised at how much there was the next clean night), I'm hoping the snails will remove some silica. So perhaps the extra nitrate, the missed clean etc etc have just overwhelmed the Coraline algae at this time as you say.
Interestingly, I do have a small area of Coraline algae that has gone white. I wasn't worried about it and thought it was unrelated. It's only a small patch about 1 cm x 1.5 cm. I have quite good current (400 ltr tank with 3 x 55W pumps and 2 other smaller power heads) sometimes when I clean the pumps they go back at a slightly different angle and sometimes cause a vortex in a certain spot. That spot happens to be where the Coraline algae has gone white, I assumed it was because it was exposed to air, actually I think it is, but I still thought I'd mention it.
One thing I had wondered about and it goes against what everyone tells me I know, but I wondered if it was possible to have too low a phosphate and nitrate level. I have noticed that when I clean the glass and disturb the sand (I can see a change in phosphate levels for a few hours), how well everything does for a few hours (that's not to say it doesn't do well anyway, it just seems to do super well after I have cleaned).