Learn Swedish: Algerna växer dubbelt så snabbt i dessa akvarier. - The algae grow twice as fast in these aquariums.

You only need som (as) when you explicitly state what you are comparing the subject to, like dubbelt så snabbt som i havet (twice as fast as in the ocean). Because the thing we are comparing the aquariums to is already implied by context, som is left out.

Why is it snabbt with a -t instead of just snabb?

Here, the word describes how the algae grow (växer), which makes it an adverb. In Swedish, adverbs are formed by adding a -t to the adjective. The multiplier dubbelt så doesn't change this rule; it just modifies the adverb.

Why is the noun in the indefinite plural (akvarier) rather than the definite plural (akvarierna)?

Because it follows the formal demonstrative word dessa (these). A strict rule in Swedish grammar is that denna, detta, and dessa must be followed by a noun in the indefinite form. If the sentence used the more everyday de här instead, you would need the definite form: de här akvarierna.

Akvarier looks like an unusual plural. What is the singular form?