English, Latin, or Malay pronunciation of betta fish

The genus name of the aquarium fish Betta splendens derives from the Malay word "ikan betah." The common name of the fish is also "betta," which in English we'd pronounce with a soft e. I often hear the pronunciation "bay-tuh," for both common and genus name, and just as often hear fish fanciers loudly object and insist on the soft e. But would it also be correct to use a Latin pronunciation for the genus name, which I think (not sure) would be closer to "bay-tuh"?

The answer is all of the above, any of the above, or none of the above. There is no single set of rules for the pronunciation of taxa, and no single interpretation of such rules as some have attempted to compile. As Michael G. Simpson notes under "Pronunciation of Names" in Plant Systematics (2006),

Although scientific names are universal, their pronunciations may vary from region to region, especially between different countries. For example, European pronunciations are often different from those of most American botanists. There are no firm rules as to how scientific names should be pronounced. Very often, pronunciations are influenced by one's native language. One should be flexible and adaptive with regard to pronunciations, as the overriding goal is communications.

Broadly, most English-speaking practitioners of science, medicine, law, architecture, and other disciplines follow traditional English pronunciation of Latin for the Latin and quasi-Latin terms in their fields. There are those who push for what is known as Reformed Academic Pronunciation, a system devised in the late 19th century which is supposed to be closer to the classical pronunciation, and which is closer to the way the words would be pronounced in most continental European languages. William Stearn favors the latter in his widely cited Botanical Latin (1983), but as countless papers, guides, and appendices note:

• Professionals not only use different pronunciations from one place to another, but do not consistently follow the same system themselves.
• People tend to pronounce names based on how they first hear them, as opposed to a particular system of pronunciation
• English speakers don't agree on how to pronounce English; how would they agree on how to pronounce any other language or pseudo-language? (Yes, I am one of those people who is always going on about bruschetta— it's Italian, not French or German.)

Zoologists and bacteriologists moved away from requiring a classical basis for names some time ago, and botanists have moved in the same direction, so perhaps there will be less debate in the future.

I can't comment on Malay pronunciation. (It's unclear to me how much direct influence the Malay pronunciation of betah has had on the English pronunciation of Betta/betta.)

"Bay-tuh" is not really a more Latin pronunciation of Betta. There are various systems for pronouncing Latin, but in nearly all of them the vowel in the first syllable of Betta would be a monophthong (a pure vowel sound), and the sound found in English "bay" is usually a diphthong [eɪ].

Betta obviously did not exist in the Classical Latin era (the earliest date on the Wikipedia page is 1910). But based on the typical correlations between Latin spelling and pronunciation, a Classical-Latin-style pronunciation of Betta would most likely be something like [bɛtta]. This is about the same as the pronunciation in what is called "Ecclesiastical" Latin. The vowel in the first syllable would be similar to the vowel in English words bet, better. In theory, the spelling Betta is also consistent with a pronunciation with a long vowel in the first syllable (which in reconstructed Classical pronunciation would be something like [beːtta]), but it was somewhat unusual, although not impossible, for a long vowel to occur before a double consonant in Latin.

In traditional English pronunciation, there is no question that the double consonant would regularly call for the use of a "short e" sound /ɛ/ (as in bet): we see this pronunciation used in English even in words that are thought to have been pronounced with a long vowel before a double consonant in Classical Latin, such as stella.

There are a few Latinate words/terms/names aside from betta where you might hear a "long" vowel pronounced before a consonant that is written double, but these pronunciations are all irregular (and for this reason, some people might criticize them as "mispronunciations"). The words I know of that have irregular pronunciations like this are camellia, ampullae, buccal, medulla, corolla.

The famous ichthyologist William T Innes says in the Innes book that the correct pronunciation is Betta - just as it looks and he specifically says it is not Bayta. My father and I bred and raised them when I was a child and all of the fish people in the 1970's pronounced it correctly. It was not until the 1990's when people stopped reading as much and began to degenerate that I begin to hear it pronounced incorrectly.