In fishes, the blood that passes through the heart is

Fishes have a 2-chambered heart with an atrium and a ventricle.

Single circulation is observed in them ; Gills$\to$Oxygenated blood$\to$Body tissues$\to$Deoxgenated blood$\to$Heart$\to$Gills.

So, only deoxygenated blood passes through the heart of fishes which is why it is referred to as venous heart.