Could someone please explain how to do frog breeding? I'm especially confused about land frogs vs uw frogs, do they both drop spawn? Thanks

The land adult frog works according to what you spot in other gardens.  For every complete set of (tadpole / froglet / underwater adult frog) you spot, you will be due 1 land frog.  The applicable totals can be found on your back wall.  The first and every 5th land frog will drop frogspawn if you have an empty plate out when it's spotted.

The underwater adult frog works according to what has been spotted in *your* garden.  The applicable totals are listed beneath your garden.  Every tadpole spotted in your garden will return as a froglet.  Every froglet spotted in your garden will return as an underwater adult frog.  Again, the first and every 5th underwater adult frog lays frogspawn if you have an empty plate out.

To start the cycle, you will need to attract a land frog to drop frogspawn.  You then need to go underwater and attract tadpoles to the frogspawn.  Once you get some tadpoles spotted, you can put out a low plate of berries (still underwater) for froglets.  After that, a high plate of berries underwater will bring underwater adult frogs.

If you always have at least 5 more tadpoles spotted as underwater adult frogs, you will always be able to get a plate of spawn from an underwater frog (and, thus, never need to go on dry land to get more).

Love this explanation thank you!! (I'm hopeless at this game LOL)

From this then if you want to do the bottom dwellers how does that work? I am considering doing the bottom dwellers or the billy goats next and can't work out either of them despite having read some information. So any help would be appreciated.

Thank you, Rachel! That helps a lot.

@Kim -- as long as you are due underwater frogs, they will visit anytime you have a plate of high berries out underwater.  The bottom dwellers require berries be out for the guppies to color the coral, anyway, so you would just need to make sure you are due the correct number of frogs and plant / remove the dulcia somniflora (frog deterrent) accordingly.  (Or use low berries for the guppies and get your froglets without the deterrent and then put out another plate of high berries for the frogs.) 

Thank you Rachel ... that is the clearest anyone has explained the process .... I have time and again been frustrated by the lack of u/w frogs in my garden .. have wasted so many seeds and berries.

I think the best thing for any of these Harder parts of the game is to complete the fairytales first so you have that 3rd table to assist with things if garden space allows for a 3rd table.