Lev 11:9 These you shall eat of all that are in the waters: whatever has fins and scales in the waters, in the seas, and in the rivers, them you shall eat.
Tuna in general do not have scales
Lev 11:10 And all that have not fins and scales in the seas, and in the rivers, of all that move in the waters, and of any living thing that is in the waters, they shall be an abomination to you.
Tunas have fins and scale (although the scales are small) and therefore are clean. So yes, according to the bible Adventist can eat most Tunas, but a whole food plant-based diet is better.