Originally Posted by OnePsychLynne
If I understand you correctly the reason the sandbed must be under 1" or over 4" is that under 1" there isn't an overabundance of nutrients that the bacteria can't break down and over 4" there are enough denitrifying bacteria to break down the nutrients that get trapped in the sandbed without there being a negative impact on the entire system?
Almost. At over 4'' there are different TYPES of bacteria in place which function in different ways. One of these are denitrifying bacteria that lower Nitrate by turning it into Nitrogen Gas, which leaves the system naturally. But yes, at under 1'' the bed remains stirred well with sand sifters and nutrients generally do not get trapped.
I am concerned that you will not be able to successfully have a 4'' sand bed with Horseshoe Crabs. Their constant stirring will prevent the proper oxygen levels from being attained deep in the bed.
I hate to say it, but I think you would be best removing the Horseshoe Crabs or staying below 1'' depth.
In the next day or so someone is going to suggest that you use light defuser (eggcrate) to separate the sand layers so that they can not be stirred by the crabs. I have seen this idea cause serious problems when used in conjunction with a deep sand bed using a PLENUM. The light defuser slows water flow down to such a serious degree that calcification can occur and the bed can bind into rock. It isn't pretty. I have no experience when the light defuser is used without a plenum.
I would expect it to take 4 to 6 weeks for the system to stabilize. Yes, I would continue doing partial water changes until ammonia is zero.
Still concerned about alkalinity. Those dip strips aren't worth much.