I agree with CFC. Small can be beautiful, but you have to have the right fish. If there is a larger fish you like, see if you can get a dwarf variety or similar fish that is smaller. I have a 25 gallon tank myself and its quite heavily stocked but I've had no problems because I've been very careful about the type of fish I have, sticking mainly to small fish, except a pair of pearl gouramis and a dwarf plec.
I like to think of my tank as being a public aquarium in miniture

Most of the fish in it are small tetras, cherry barbs and pygmy loaches. A school of tetras swimming through the undergrowth can look quite spectacular - much nicer (IMHO) than couple of larger fish in a tank that is obviously not big enough for them.
Incidently, I moved my SAE from the 25 gallon because being a tall tank, it didn't have the length he needed to swim in, plus he needed some friends and he will grow to about 5", all being well. Since moving him into my 30 gallon long tank he's put on weight and length and is a different fish! He can stretch his fins and take long swims the whole length of the tank (which he frequently does) or even do a circuit. I suspect a shark, if you got one, would have the same problem in your 25 gallon. A red-tailed shark might be theoretically OK, but consider the length of your tank and the actual swimming room since all species of FW shark like to swim, much like my SAE.
Tetras, OTOH, are fairly sedate fish and tend to "hover" in the water a lot, with the occasional dart. They are far better suited to a smaller swimming space, perhaps with plenty of plants, bogwood and rocks (in the wild they live in pools and streams with loads of undergrowth, tree-roots and rocks rather than open waters).