Our LFS has a BEAUTIFUL Flowerhorn on display. He is over a foot in length and is only a little over a year old. A "teenage" Flowerhorn would not even physically fit in your tank. They grow EXTREMELY fast.
Frontosas must be kept in small groups. I guess you could say they are "schooling". They will not last long if kept signally. They also get absolutely huge, just like the
I wouldn't advise a single Oscar either. While they are small now at pet shops, they grow to over a foot in length very quickly as well.
You will be VERY disappointed if you put a pair of Discus in that small of a tank. They will quickly wither away faster than any other fish on this list. They require a long tank for swimming and said tank must be heavily planted to provide cover and shelter (they are a little on the shy side) and also to mimic their natural environment in the wild. They are also EXTREMELY difficult to care for. Google "Discus diseases" and you will find a LONG list of diseases that Discus (and only Discus) are suseptible to. The worst being the "Black Discus Plague" which causes them to turn black, wither away, and die. Most of these diseases are also contagious so if one fish has it, the others will surely follow. This makes it difficult to purchase a healthy one as well since most LFS have 10-12 in stock at any given time. The kicker is Discus have somewhat of a weak immune system. Meaning, if they get overly stressed they will more likely than not fall ill. At about $40-50 a fish, it's a pretty price to pay.
Angels seem about the only viable option. They are very hardy and easy to care for. A pair would only work if they were juveniles (a tad bigger than an American Quarter). However, they will have to be moved to a larger tank as they grow. Although they don't grow as large as the others on your list, they still do grow to a very decent size much too large for 30g. What about a pair of Kribs? They are brightly multi-colored Cichlids that are very interesting to watch and stay relititively small.
I don't know how experienced you are in fish keeping, but if you are may I recommend a Green Spotted Puffer? If this is your first tank I do NOT recommend this fish. They are a "neon"-ish green to yellow color with black spots and a white underside. They are absolutely beautiful and a fun fish. After a while they will begin to recognize their owner and get very excited when you enter the room. There are a few downsides however. First of all, they will not eat a flake or pellet food. They require frozen bloodworms as well as live brine shrimp. It can definitely be somewhat of a hassle to have to keep live shrimp on hand. Even more of a hassle being they require a readily available source of live snails. Puffers have teeth "plates" that continuously grow and eating snails wears them down. The upside is snails breed very easily and quickly. All you need is a 5 gallon bucket (the ones from Home Depot, Lowe's, etc. work perfect) with cut up PVC piping at the bottom. I live in South Florida so I keep my "snail bucket" for my GSP outside by the pool (of course out of the way of direct sunlight and rain). One last note, Green Spotted Puffers are BRACKISH fish. They will require slow introduction of marine salt into the aquarium to survive. Once they are weaned to brackish water the salinity MUST be maintained at a constant level. Too much salt at once or not enough will stress your puffer out and lead to possible death. A 30g tank is a great start for one puffer, but again you may want to consider upgrading in the future. If you are looking for a fun and rewarding fish with lots of personality, and you are experienced enough and are willing to dedicate the time and resources into taking care of one, they are awesome! I had two for four years and they were the best fish I've ever had.
My last recommendation is again, only if you are experienced. Freshwater Pipefish are a BEAUTIFUL species that is surprisingly not very common in the aquarium trade. They are closely related to Seahorses and share many traits with them. In fact, they even look like a straightened out seahorse. They are a pretty hardy species as a whole, but what makes them difficult to keep is they require a constant source of live food. There are many ways to cultivate and keep live brine shrimp but again, this is just one more thing you have to care for. They can be trained to accept frozen Mysis and Krill as they grow older, but this fish will only eat food that appears to be moving. That is something you have to consider with this fish, or the puffer, or pretty much ANY fish for that matter. If you go out of town, on vacation, fall ill, etc. someone will have to be available to feed them. Like I said, neither of these two fish are for the beginner aquarist.
As I'm sure you are realizing now, it is very hard to have a "species-only" (one fish) tank in such a small aquarium. Most true species-only fish are going to require at least a 60-75g tank. These were just suggestions. Before purchasing any fish make sure to do EXTENSIVE research. Again, and I cannot stress this enough: if you are NOT experienced, this fish are NOT for you.
Honestly, IMO at the end of the day I would just keep the fish that you already have in your 30g in there until you can purchase and cycle a much larger tank. I know it's hard to resist and community fish aren't very exciting, but trust me you will have a MUCH better fish keeping experience if you house species in their correct suggested tank sizes.
Hope some of this helps!
