Having just struggled through these, I could not find enough easy to follow help, so have written this out in the hope it may help other fairies who are keen to try.
I have also written a specific help for the Starfish and will post it as soon as I've finished checking it.

Good luck and enjoy if you do the Bottom dwellers.

Basic strategy for all levels of Bottom Dwellers.

Plant 1 frog deterrent (dulcia) and go underwater.

Plant the corals in the uncoloured pots - whichever you need for the level.

Put in the rainbow rock and whichever mushroom you need for level.:I would recommend that you already have the rainbow rocks before attempting the bottom dwellers.

The following is what is needed to plant the pondweed in case (like me) you have forgotten.
Every 10th Watery Sea Dragon
10th Rainbow Sea Dragon 
10th Golden Sea Dragon 
6th Griffish 
6th Phoenish 
4th Sea Pegasus
3rd Sea Unicorn 
These will plant the Pondweed that you need to attract the guppies to colour the coral.

Plant in the correctly coloured pot the particular Pondweed for the colour you are needing. (appropriate to the mushroom you planted).

Once your Pondweed is planted, remove the mushroom and plant a forest ruby jewel (or whichever most appropriate for level)
Here is a list of guppy deterrents but in the higher levels you do not have room to plant them all. See specific instructions for the Star Fish Below.

Guppy Deterrents:
Red - Ruby or Amber
Orange - Amber
Yellow - Sunlight and Ruby
Green - Emerald and Amber
Blue - Sapphire, Sunlight and Ruby
Indigo - Moonlight, Emerald and Amber
Violet - Amethyst, Sapphire, Sunlight and Ruby

Put out 2 tables of organic berries for the guppies to colour your coral (no other mushroom really necessary as guppies come quite happily)

Here is a list of the needed colours for the specific Bottom Dweller:

Lobster ~ Red

Octopus ~ Orange

Catfish ~ Yellow  plus Orange to deter Lobster.

Stingray ~ Green  plust Orange to deter Lobster & Yellow to deter Octopus

Spider Crab ~ Blue plus Orange to deter Lobster, Yellow to deter Octopus & Green to deter Catfish.

Shrimp ~ Indigo plus Orange to deter Lobster, Yellow to deter Octopus, Green to deter Catfish & Blue to deter Stingray.

Starfish ~ Violet   plus Orange to deter Lobster, Yellow to deter Octopus, Green to deter Catfish, Blue to deter Stingray. One actually could use Indigo as well to deter the Shrimp, but there is no room, so unfortunately one has to deal with a lot of Shrimp visiting as well.

Just a little word of encouragement. You will need to plan on doing the starfish at least twice. I have seen very few that have managed it in one go, so try to keep your spirits up and your sense of fun and realization that this is a game in high gear. This is a difficult level so enjoy it. The problem is that there is not enough room in the garden for all the deterrents to be planted, so the best one to leave out is the highest, which is Indigo, and then one tends to get many Spider Crabs visiting. Then it is down to luck.

Once the corals are coloured in remove the pondweed and remove the frog deterrent. In the higher levels when you are short of space in the garden, use two low plates (no high plates) and then you don't need the frog deterrent as it is only the adult frog that one needs to take care doesn't come so he is still in line to lay the spawn for you.

Put out a High plate of Superberries for frog, empty low plate for spawn. I would HIGHLY recommend that you use Superfood.

Replant frog deterrent once spawn is delivered .

Plant in Supeshroom to deter Mermaids.

Wait for the critters to arrive. Good Luck and enjoy.

Thank you. This is so helpful, I may even try them now.

Great post, Liz :)

Thanks Tracey.

Does the birdbath act as food for ALL birds ? I know nothing at all about the birdbath....and how does it attract chicks ..I saw one earlier that had several birds and chicks spotted on it.
Is there a forum thread about the birdbath ?

It attracts all of the adult birds which can have chicks - so the adults of all of the birds listed in the Chicks section - and will attract them even if you haven't already attracted those birds.  It will also attract any of the chicks which you have already had spotted, without you needing to set up for them again.  The birds/chicks come at random, there's no way to predict which will come.  And the chicks don't give alerts.

You set up the bird bath when you're underwater; the mermaid leaves water in it when she's spotted (the water you get reflects the food which the mermaid was spotted on - superwater if you leave out superfood, organic if you leave out organic, and so on).  You'd then have to go back onto dry land to get the birds/chicks spotted otherwise you'd just get mermaids to the bath.

Do you have to complete the combo's before attempting the bottom dwellers??? xx

No Rose. 
  It is impossible to complete the BD without complete the Rainbow of Fish.   
  I know many that have done the two (RofF and BD)  side by side to Indigo,   but then the garden space and time restrictions only allow to  finish RofF.

The two challenges are not dependent on the Combos.

Great post by Liz, I too might give them a go during the long Winter dark evenings. To take the time to write these help guides is so appreciated, thank you xxx