I agree with 737mech/George on moving them to a little smaller tank. With the fish sizes and quantity, you will need RO/DI water in huge quantities in a relatively short time. Assuming that you can produce/buy a large amount of water daily, do you have extra storage for the water that will be needed as the ammonia spikes,etc? Are you also planning on removing the sandbed before the hypo and how deep is it currently?. While I really do prefer to do intank hypos too...(or at least keep one hypo tank with liverock/sand in it), the first time you drop the salinity on an existing tank, you will get a large ammonia spike from the extra load imposed while the sandbed and/or liverock animals die. The bacteria to do the ammonia cycle are not apparently harmed by the drop, but the extra load of all the sandbed/liverock die off is much more than it will be able to handle. When I first hypoed an existing 45g with rock and sandbed, it took about 2 full water changes over about 1 week for the die off to finish and the cycle to catch up again. After that, (and since I didn't need to add copper), it was normal maintenance for the duration of the hypo stay. I didn't need to change water daily,etc. However, it also obviously put additional stress on the animals during the conversion, so if they are very sensitive to elevated ammonia due to illness and/or species intolerance, then it will be even more difficult. Also, with only the sandbed remaining in the tank in your case (assuming it is...), it won't be enough to offset the ammonia produced if you had the live rock in the hypo too. So you will need the ability to produce/change large quantites of water as above. In your case, I'm not sure what I'd do to hypo that much livestock of that size, but removing rock *and* sandbed would be the best bet *if* the sandbed is very shallow. With a deeper bed, I don't know that I'd try to disturb it with the animals in it. If you could remove the rock and put in containers with aeration (and heat/cool if needed) and remove the animals temporarily, you could take out the sandbed too. I know this doesn't solve your problem, but just wanted you to know the pros/cons of hypo-ing the display itself --i love it, but it's a real pain the first time and there's not much real advantage to using a sandbed only tank if the rock is removed also., just go bare tank and be prepared for the few weeks of large water changes... :sad:
