Need a few thoughts on the desired suction pressure/temp on salt water brine system..

Hi all,
We are currently replacing a ageing brine system on a large commercial fishing vessel and was wondering if anyone had some thoughts on the required suction/pressure temps required. The single system runs 6 x brine tanks. Seawater / salted brine with a spiral type (stainless 1") evaporator with a pvc outer casing forcing water from the bottom and out the top. Sizing the system load is difficult because on one hand you can have 2 tanks loaded with 2 tonne of 18c fish and once down to temp the system is only required to hold the product. The problems arise with the coil temps in regards to icing over the pipework, once a layer of ice forms over the evaporator coils it's very difficult to remove when the water temp is a salted -.5 brine. I am currently designing the system with a -5c temp at the evaporator with a 2k superheat.... Any ideas from anyone with similar experience with these type system.
cheers!