I'm trying to get Frogspawn to Complete the Bottom Dwellers but don't really understand this can anyone help please ?

You have to start with land frogs to get your first spawn.

For every set (tadpole, froglet and UW frog, including ghostly frogs) you spot in other gardens, you will be able to attract a land frog to your garden.

You already have 14 land frogs spotted in your garden, so your next one (15th) will give you spawn.  You need to check how many sets you have spotted in other gardens, they have to be at least 15 to be due a land frog.

Just keep in mind that starting Bottom Dwellers is the last step for this but it takes a while to get all bottom dwellers. It is also mentioned in the Wildlife guide:
"This is the most difficult of the Pond challenges, so you should complete all previous ones before attempting any of the wildlife in this category!"

Like Marike explained since you never done frog breeding you have to go on land, to get one frog and then you can get frogspawn. Afterwards you got this, you will go underwater and then you can get lobsters.
But before doing this, you would need some Watery Seadrogons for a pondweed. If you have the red pondweed, then you can attract red guppies which will paint the coral in red.

Ok thanks for your help sounds difficult have just completed the Rainbow of Fish

I just found the website again about Bottom Dwellers.
You can have a look to get an idea how to do it :-)

https://freedom-fairies.weebly.com/bottom-dwellers.html