Dman, IMO the reason you didn’t see any changes in the first tests, is probably due to the clams still being in the acclimation phase, just like a lot of fish don’t start eating till a few days after introduce to a new tank, your results look very promising but I would be cautious of introducing to many clams as they may not have enough nutrients to survive.
I am ready to try this but I’m only going to start with 6 clams, scrub them real good, then acclimate them in a separate tank for a few days with several water changes from the man tank, this will allow any water that comes in them, to slowly dissipate, if they appear to be doing good, they will then go on a sand bed in to the refugium, with plenty of hermits a n snail, in case one dies they hopefully get eaten up right away, I would then monitor the nitrates and see if they change and depending on these results, try to determine if more clams should be added or not.
The goal will be to get the nitrates as close to zero as possible and try to keep a balance where you nitrates are low but still have something for the clams to feed on.
These are just my thoughts, if anyone has any other ideas, good or bad, don’t be shy, cough them out, that’s the only way we are going to figure this out.
Dman
on your test, I appreciate your effort, Keep us posted on what our Canadian neighbors are coming up with.