WMG / H2O: Just Add Water

Byron and Miriam were abducted by aliens at some point between season 1 and season 2.
And the aliens probably erased everyone's memories of their existance as well.

Byron secretly knew that Emma was a mermaid.
He may have seen her mermaid form during Emma's father's birthday party, while Emma was under the influence of the full moon. Figuring that it was supposed to be a secret he didn't tell anyone. Although it was only after the episode where he asks Emma to help him beat Zane in a swimming competition that he figured out how the transformation works.

Julia was the only one of the previous three mermaids who kept her powers.
Miss Chatham reveals that she gave up her powers, and it's implied that Gracie did so as well. However, we never learn if Julia did so as well. Out of Cleo, Emma and Rikki, it seems that Rikki is the one who enjoys being a mermaid and having special powers the most. Since Julia apparently was somewhat similar to Rikki, it's possible that Julia chose to keep her powers.

Emma, Cleo and Rikki are related to Lucia, Hanon and Rina respectively.
This is one of the reason why they were destined to be Teen mermaids before the show's beginning, since the said latter 3 could be their Japanese cousins. Another is Aqua Regina already had it planned for them after the 1st 3 Mako Island Mermaids.

Elliott knew the girls' secret (almost) all along.
When Cleo made excuses about her diary and her tail, that was enough to convince Kim, but not Elliott; Kim hadn't had her shower suddenly start hailing on her. Emma might have tried to explain that or she might not have even known it had happened, but Elliott knew the truth. When she refused to go on vacation, he wasn't just mad at her for not going; he was mad at her for not trusting him and their parents. But he decided eventually to leave her alone and suggested to the family to go someplace dry so she could be more at ease. He just never brought it up because he knew it would just cause problems, the way it did when Kim 'caught on'.

Becoming a mermaid makes you selfish and petty.
When Charlotte said that Cleo, Emma, and Rikki only care about themselves, she wasn't wrong. The girls show themselves to be petty, selfish, spiteful, and manipulative over and over again. On the other hand, Charlotte was actually a pretty decent person who seemed to be genuine in her efforts to have a relationship with Lewis... at least, until she got mermaid powers, at which point she became as bad as the other girls. When she's Brought Down to Normal, she acts more like her old self again.