Spartan Alex recently posted a quick bit of errata in the Spartan forums for the Oannes Heavy Carrier, piloted by the Aquans and found in the Return of the Overseers box set.
The Oannes taking up two Tier 1 slots has come up a lot in the forums since the models release. As correctly surmised by some, this is to prevent some rather over-powered builds at Patrol Fleet level, and represent the fact this is a new ship type for the Aquans, and not fully integrated into their command structures – thus less generally available for smaller engagements. A Grand Admiral is less likely to grant lots of Tier 1 forces including brand new ships to a junior commander requesting them for a small skirmish than for a large battle.
To represent this, please adapt the Oannes Taking two Tier 1 slots to be applicable ONLY at Patrol Fleet level. In larger fleets, it will take only one Tier one slot, though it still takes two carrier slots in all cases.
The errata applies as of May 21, 2015.