Here are some factors in what fish are imported:
1. Seasons affect mostly SA fish, dry season from August to January most of the time is when we will see an abundance of fish from Brazil. Colombia and Peru it can be off several months as the west side of the continent has slightly different weather patterns.
2. Political factors influence fish out of Africa, it is such a unstable region that different exporters have come and go in countries like Zaire, Tanzania, Zambia. Fish can become commonly available then suddenly disappear when governments change.
3. Legal factors - there are still a very large list of banned fish out of Brazil. There are also treaties between Brazil and its neighboring countries to not export any of those fish. The US has a treaty and can actually confiscate any banned fish from any country if the wildlife inspector chooses to. Countries like Colombia and Peru have collecting seasons for certain fish like Altums and Arowanas, outside of those months they are illegal to catch.
4. Financial factors - Exporters want large sums of $$ up front before they will ship any fish, and especially illegal fish. Many people, including myself, have lost thousands $$ to try and import the 'rare' fish only to be told the fish are no longer available or the exporter disappears with the money. Like Wes said there are minimum orders to be considered, with 10 boxes the usual minimum. Do I really want to bring in 10 boxes of the same fish when everybody that screams about wanting them all of a sudden doesn't want to pay the price for them?
5. Competition - it is very common for exporters to collect lets say 200 Tigrinus and then offer them for sale to importers. Of course we are all going to jump on them to bring some in. Next thing you know 5-6 vendors are listing them for sale, all of a sudden now the fish is considered 'common' and no one wants to pay the going price. Now we vendors have to start slashing our prices because we don't want to hold on to 30-40 tigrinus and be eaten out of house and home. They may be the only 200 Tigrinus offered for the whole year, but because they all come in at the same time the perception is that they should be cheaper. In the end only the exporters make money while the vendors kill each others profits by price drops and then no vendor wants to bring them in again. Now the cycle repeats itself 'wow Trigrinus is very rare again'!
My concluding thoughts are this - there isn't really any fish that I would consider rare to find in the wild except for what is already on the CITES list. Either legal or seasonal issues control the availability of the fish or the vendors have to consider whether or not certain fish are worth taking a financial risk to bring in. I get requests for many specific types of fish, but whether they are profitable for the fishermen to spend the time to collect and sell to exporters is another story. For example take the striped silver dollar, Mytennis fasciatus - a fish only seen a few times in the US past couple years. They are only found in one river in Brazil, fishermen have to travel a long way to catch these fish with no other profitable fish in the same area to sell. I ask my supplier to find these fish and it takes them 6 months to convince the fishermen to make the trip. They brought back a total of 6 boxes, I got one box and the other 5 to my knowledge went to Asia. They were only on the availability list once this entire year and I got some in. Would these be considered very rare fish? Heck yea, I know how hard it was to get just a box of them. Why do I still have some left in my tanks? With the large investment I put in to not have the demand they deserve I wouldn't bring them in again even if it takes another year to see them again. By then are they going to be considered 'rare' again?
Its anyones guess what will be the 'hot' fish of the current time. For me, I have decided that I am going to bring in fish that I think are not common and beautiful in my opinion. That way if I don't sell certain fish I still get to enjoy looking at them
