Weird situation--Undisclosed additional pets?

I'm a new Rover sitter. I just ran into a kinda odd situation on literally my first overnight.

The household I'm currently sitting for didn't mention that they have exotic pets (two axolotls) that need minor additional care. I did do a meet and greet in which I was in the same room as their tank, but, having inhumanly poor powers of observation, I didn't notice their tank, and the owners didn't mention it.

I noticed the tank yesterday and messaged them to ask if there was anything I should or could do for the axolotls; they said they hadn't wanted mentioned them so as not to "burden" me with it, but yes, they are supposed to eat every two days. The stay is four days so obviously I said I would feed the poor guys!

I shared this with some friends today just as a funny story, but they told me I was being a pushover and I ought to insist on more money when the owners return tomorrow.

I don't know about that--I feel like springing this on the owners right after they return from a trip would be really inappropriate, especially since it's partially my fault for not noticing the tank when I had the chance + the axolotls are very little extra work. Everything else about these clients has been really nice and easy.

Am I being reasonable, or am I being a doormat? If so is there a tactful way to do as my friends advised?