Here's a candid question from my friend Bob: "I just got a 20-gallon tank and was thinking about putting in about a dozen Neon Tetras, six to eight Mollies, a Plecostomus and perhaps introducing two to three Angelfish afterwards. Is this an acceptable plan?"
My answer was, "Bob, you're inviting disaster into your aquarium! You will be defying at least four major guidelines in the book of 'good fishkeeping'!"
Defiance 1: "The one-inch-per-gallon rule is baloney!"
Although not a strict rule, the one-inch-of-fish-per-gallon-of-water guideline is often a takeoff point for fishkeeping hobbyists aspiring for a stable bioload. However, factors like efficient filtration, fast-growing fishes, and aggressive or territorial breeds totally overhaul this "rule" giving rise to setups with more than one inch of fish per gallon, or with just a pair of two-inch fish for a 25-gallon tank.
In Bob's planned 20-gallon setup, 12 Neons, 8 Mollies, 1 Pleco, and 3 Angels, even if they're all less than an inch each, will surely overload his tank habitat, no matter how efficient his filtration is. But let's say that on top of a truly efficient filter, he can be diligent enough to make regular water changes, his next problem will be what water parameters to maintain.
Defiance 2: "What natural habitat? I'll give them their natural habitat!"
One of the primary objectives of fishkeepers is to provide a habitat that is closest to what the fishes have in the wild. The community that Bob is looking at putting together cannot occur naturally because the fishes he has chosen do not thrive in the same water conditions.
Although all the chosen fishes can be made accustomed to a common water temperature (around 77 degrees F), Neons prefer water that is slightly soft, with pH between 5.0 and 7.0. Mollies, on the other hand, live in slightly hard water with pH of 7.5 to 8.5.
In other words, either the Neons or the Mollies will suffer stress and eventually fall ill if brought together in one community tank because the water composition is outside the range ideal for them.
So, if Bob removes the Mollies from the equation and maintains a water composition ideal for the rest of the fishes, will the Neons, Pleco, and Angelfish be fine together? The answer is, again, "no."
Defiance 3: "They don't look aggressive to me!"
Angelfishes belong to the notoriously cranky group of cichlids. Highly territorial and potentially aggressive, Angels (an oxymoron?) are known to be "stealth" attackers. Swimming casually and almost in a delicately gentle manner, they can snap at a prey in a flash and swallow whatever fits their mouths. Full-grown Angels are known to feast on Neons.
So, maybe Bob can chuck the idea of adding Angels later on. Will his tank be safe now with the dozen Neons and one Plecostomus? Unfortunately, the Pleco may pose a whole new problem.
Defiance 4: "They're just sooo cute!"
The Plecostomus is one of the fastest growing catfishes in captivity. A "cute" three-inch Pleco can double in length in six months and grow to a "horrendous" adult size of close to two feet in a couple of years. Their phenomenal growth is accompanied by their equally phenomenal rate of excretion that can easily overwork standard filtration systems. A Pleco will, therefore, be okay in a 20-gallon tank only in its first few months as a juvenile.
In other words, Bob's 20-gallon tank is down to just a dozen Neons. That's no fun, right? Well, a 20-gallon tank can't really support a lot of fishes, and frankly, losing some to not having an acceptable plan is my definition of "no fun." Bob can still have a fun tank by choosing a combination of community fishes that are attractive together and thrive in a habitat that will remain stable indefinitely.
Here's one suggestion: to join his Neons, he can add a pair of Rams (Ramirezis may be cichlids but they're one of the more sedate ones), and instead of the Pleco he can choose a pair of Corys (corydoras) or Otos (otocinclus) that remain small even as adults.
Add to that some diligent monitoring, regular water changes (or better yet, an EcoBio-Block that conditions water so that changes are hardly necessary), and efficient equipment, and Bob can be well on his way to implementing a truly acceptable plan.