Learn Portuguese (Portugal): Os terrários são pequenos ecossistemas onde podemos pôr florezinhas. - The terrariums are small ecosystems where we can put little flowers.

When using the suffixes -zinho or -zinha to make a plural diminutive, you don't just attach them to the singular word. The rule is to take the plural of the base word (flores), remove the final -s (leaving flore-), and then add the plural suffix (-zinhas). This gives us florezinhas. Note that in informal everyday speech you might hear people bypass this rule and just say florzinhas, but standard grammar requires florezinhas.

Why does the adjective pequenos take the masculine form if ecossistemas ends in "-a"?

Just like the word o problema, many words of Greek origin ending in -ema are masculine in Portuguese. The root word here is o sistema (the system). Because ecossistema is masculine, the adjective modifying it must also be masculine, resulting in pequenos ecossistemas.

Why does the verb pôr have a circumflex accent here?

The accent is a "differential accent." It is used entirely to distinguish the infinitive verb pôr (to put) from the preposition por (for/by/through) in writing. Without it, sentences could easily become ambiguous.

Is onde the right word to use here, or should it be aonde since we are moving flowers into the terrarium?