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Based on your description, I found an Anime made by Nippon Animation, the people behind Future Boy Conan. It's called "Blocker Gundan 4 Machine Blaster", in Italy it's known as "Astrorobot Contatto Ypsilon". The protagonists are 4 Mech Pilots who fight against an invasion. It's also pretty old, I'm speaking of the 70s.
I'm sorry, but it's not the one. If it was not for the creatures I saw (the charred men were quite creepy) it could have been even a non-serious anime: the boy had the classic "Opaque Nerd Glasses" complete with spiraling lines and his mech was literally a metal fish (like a big metal carp). In the episode that I saw they fought against against a giant tentacle monster inside a tunnel: it was pale, with smooth tentacles that crawled and whipped at an impressive speed and with some discoidal central body that the younger girl (a chubby short girl that lost a tooth during the fight) tried to attack with her bug-like mecha (another funny-looking mecha, it was roundish, with two eyes and two pick-shaped frontal appendages). This anime must have had some notoriety, otherwise none couldn't have been bothered to translate it in Italian.

Hi guys, could you please help me with this one? Twenty years ago I watched an episode of a strange anime: the protagonists were four mech pilots, two boys (one of them was a short boy with thick glasses and manned a fish-like mech) and two girls (one of them was a little girl that manned a scarab-like mech), and there was some race of humanoids that in Italian were called "Signori degli Abissi" -it means "lords of the abyss"- (in the episode that I saw they looked like featureless charred humanoids, which for some reasons turned to dust starting from their head). It had the graphics of Future Boy Conan. Thanks in advance!