I have kept tropical fish for 40+ years. I remember my first tank, a 3.5 gal tank with 2 Cherry Barbs and a Bronze Cory catfish. I was happy with that tank for about 2 weeks, and then I needed a bigger one. There were a few aquariums in the basement from years past, so I dragged out a 20 gallon.
Chrome trim, slate bottom, this was the Cadillac of aquariums at the time. Stainless top with 2 incandescent light bulbs (careful not to splash water on it, it was prone to arcing house current, and could give you quite the tingle) . I was so proud of that tank, and set it up with great care and was diligent with maintenance. Looking back at that tank, it is like looking back at a Model T. Sure, it got the job done, and no one had better, but comparing it to what we have today, wow.
Technology has made aquarium keeping easier, safer and more rewarding. We now have tanks in any shape or size a designer could fathom, that will fit into any décor, from rustic to futuristic. New materials with greater strengths and new adhesives to put these materials together have made for some very unique aquatic environments. That 3.5 gallon tank that was too small for me is now finding its way back into the hobby.
LED lighting to reduce overheating small tanks, tiny heaters that keep them warm at night when our thermostats let the house cool down, micro filters that are easily hidden and don't make buzzing or gurgling noises. These are some of the advancements that make smaller tanks easier that they were 40 years ago.
But these advancements also present new problems. With oddly shaped aquaria, it becomes harder to get into the nooks and crannies to clean. The smaller sizes make filtration and heating more of a challenge. Finding appropriate animals for these tiny tanks is also difficult. Fish need space, especially if you want to put more than one in the tank. They need room to get away from their tankmates, and room for oxygen to get into the water, and for waste to be processed out.
Some of these tiny tanks are perfect for bettas (Siamese Fighting Fish), which solves a lot of the issues. But too many have pictures of a dozen fish swimming around in a tank that would be over capacity at 4. So someone buys one, and loads it up with the pictured fish, and a few days or a week later, everything dies. When they seek out a fish expert and ask what went wrong, and get the sad news that the tank they bought will never house the fish they want, another tank ends up in a garage sale.
There was a recent trend to keeping shrimp in these micro tanks, which can be very interesting. They aren't fish, though, and while they are unique, they don't have the same wide appeal that fish swimming around a tank do.
Small tanks can be great, but make sure you understand all the limitations they bring. The best way to avoid disappointment is research.