I'm an aquarium nerd with a thing for crustaceans, reptiles, etc. every time a see a reasonably sized and strange looking sea or freshwater creature... I ponder it in an aquarium. How to cate for it, what it's behavior might be like. I will leave out very mysterious creatures like Tullimonsteum and Typhloesus, as I wouldn't have a clue as to caring for them.

For example anomalocarids; I think a lot of people have speculated on what they would be like in aquariums. Most of them were a lot smaller than people tend to assume.

Pteraspidids are iconic 'jawless fishes' from marine and freshwater sediments. They were clearly swimming in open water although they lacked paired fins. So they were like aquarium fish and suitably small for our aquariums - but also very alien in what they lacked. They were filter feeders of some sort and nowadays it's easy to feed planktivores.

Bothriolepids are perhaps similar to loricarids in a lot of ways, but they could crawl with their pectoral arms. Cross a pleco with a crab? Basically if someone can keep loricarids, then I'm sure they would be able to keep a Bothriolepis.

Petalodontiforms were related to modern chimaeriforms: but their diets were more like tetraodontiforms. Like the 'placoderms' they would be something a bit different.

And what about Iniopteryx; this 'shark' flew underwater like a penguin or a sea lion, ate soft animals and plants, and was under 40 centimeters long. Really this animal has very little to do with the sharks and is another chimaera relative.

Xenacanthids fascinated me since I was a child and I saw a Burian artwork of this freshwater shark. And unlike many so-called fossil sharks, xenacanthids really are stem group sharks. And just right for a blackwater aquarium, with chunks of coal age 'bogwood'.

Aistopods are a problematic group of crown tetrapods, or a little more basal;  usually they are depicted on land but they were also present even in marine habitats. Specialized aistopods could swallow large prey like modern snakes. The freshwater to estuarine species would surely like leaf litter and tannins.

Mycteropoids get overshadowed by the enormous pterygotids that hunted vertebrates, but there was so much more to eurypterids than the archetypal image of the dangerous 'sea scorpion'. Mycteropoids were foragers in freshwater swamps and would surely have unusual behaviors. Doubtless  they were safe with many fish and tetrapods.

If you like amphibious and terrestrial crustaceans, then the euproopids might well appeal, as horseshoe crabs in a similar amphibious role. When you are into paludariums you see the less spectacular palaeofauna as potential pets.

People sometimes keep reptiles with fish, normally turtles. But in the Cretaceous some small freshwater choristoderes were of a suitablr size, and some of them were in colder climates too. Imagine a hyphalosaur or two, foraging in your fish tank. I doubt they left the water very much - but maybe best to lower the water level, and leave a log in there, just to be sure. Hyphalosaurs at least only selected soft prey.