You could argue that Lumineon is just "another fish-like Water-type Pokémon"... despite being the only of its kind in Gen 4
For reference, I think Gen 6 didn't receive such Pokémon.
I mean, you could arguably count skrelp as one (sea dragons are pretty much fish), but I see what you mean.
I think it doesn't help that even if it's the only fish-like water type introduced in gen 4, it isn't the only one in Sinnoh, having to share water with goldeen, magikarp, remoraid and barboach. Magikarp even had its own memorable moment in the gen 4 games when they're shown in the drained Lake Valor, while finneon and lumineon don't even get to be in the water-type gym. Though then again, being one of two water-type fish in the Black and White games didn't help basculin out much.
I guess fish pokémon have a hard time standing out, as they're pretty much inevitably the most common type in the game which also happens to be a starter type, so they're really easily overshadowed. Not to mention that they're some of the hardest pokémon to work into the anime, as they'll need a body of water to swim in, while just about any other pokémon can be plopped in anywhere and do fine (the first time a fish pokémon was ever shown in the anime was basically a joke about this), so unless you need a mount to ride between a set of islands (which is a role that can just as easily go to non-fish like lapras), they're pretty much relegated to being background pokémon for underwater scenes (and that's if the anime doesn't go for actual fish instead, which it did a few times in gen 1).
Though the fact that even the non-water-type electric fish of gen 5 aren't all that used in battle even while foregoing the water type altogether (and one of them having a unique type combo), I have to wonder if part of the problem is the developers being stingy with the stats when they design fish pokémon. Maybe it's just hard to imagine a fish being all that threatening, so the people who design them don't make them all that effective in battle?