Because Nova doesn't have tentacle hair, whenever his special is charged, its hard to tell to the naked eye.
However, the bright burning of ink caused by such osmotic pressure is shown one way- through his mouth.
Being a steelhead means that unlike most non-inkfish creatures, Nova does not need a means of producing ink- his own body and especially his spit are lethal weapons (which are not allowed to be used during recreational modes.)
Think of his spit as a Toxic Mist that is LITERALLY toxic- being in exposure to it for too long can cause gradual damage. And while Nova no longer uses steelhead equipment to make steelhead bombs, it can even still be explosive- if set off in the right way- something that steelhead gear enables.
Nova's mouth, when special is charged is, burns the ink color of the ink he's embodying at the time. This makes it look like he is almost breathing fire, the fire being the color of the ink, a kind of fire seen on every inkfish when their special is charged!
His breath is noticeably hotter during this time, almost as if eaten something spicy (which he is not a fan of.) When his special is held for long it can seep through the sides of his mouth and teeth, and even through his nose!

Just a lil thing I wrote for when the special is charged~