As someone that works in marketing and sees this stuff daily, the logos aren't even close in design.
Monster energy drink does not own the trademark for exclusive use of the letter M in marketing. That can't be done.
One is green squiggly lines like a stalagtite and one is an M made to look like a devil symbol.
The fish site is not misleading in any way with it's marketing, and they do not gain any profit from trying to pretend or falsely associate themselves with the drink company.
"Infringement may occur when one party, the "infringer", uses a trademark which is identical or confusingly similar
to a trademark owned by another party, in relation to products or services which are identical or similar
to the products or services which the registration covers."
"Where the respective marks or products or services are not identical, similarity will generally be assessed by reference to whether there is a likelihood of confusion that consumers will believe the products or services originated from the trademark owner."
If the fish site starts selling energy drinks or insinuates they are endorsed by the drink company, then they have a case.