Population Unhappy Because of too much Population?

Normal. The general rule is dont let population over 18 billion or they become almost impossible to keep happy.

So remove farms and such so that total pop can hit only say 15 billion?

Edit: My pop is only at around 12b when it started to drop significantly.

Yes, usually one farm per world that you want to have a big population is enough, as when you research further down the farm tree the farm will produce more food and hence more people will be supported.

Do not even think of planting a farm on the capital world however as the initial colony is already providing tons of food A farm would inundate your people and you won't be able to keep them happy (except if you keep adding morale biuldings)

In my latest game I have +340% morale from buildings, playing the krynn whom has great bonus to morale and on 21 billion population planets morale was below 40%. cant remember what taxes i was running on, but probably 50-60.

It doesn't matter how many morale improvements you make, 21 billion people just cannot share the same planet.

It actually makes sense if you think about. No one can make everyone happy so the more people you have the more discontent people there will be.

I wish there was a way of capping population directly, rather than by not building farms. It would be interesting to have "farming worlds" just like we have dedicated industry and science planets.

Of course we have farming worlds: that's the basis of economy worlds.

Step 1: Make one of your high classes planets go up to about 16-20 billion people and then biuld economy biuldings all over with the occasional morale building to keep them happy.

Step 2: Rake in money.

But those are not farming worlds, any more than the 2 factories many players typically drop on a planet so everything else builds faster make them all manufacturing worlds. It never takes more than 3 farms to get up to 16-20 billion population, once you've upgraded to decent farms - and that's with Yor, whose best farm only makes 3 units of food.

A true farming world would have lots of farms on it, not just enough to get up to the population happiness cap and then all economy buildings. It would provide food for other worlds in your empire, much as your research worlds learn about planet improvements for the other worlds in your empire and your manufacturing worlds build fleets to protect the other worlds in your empire. Sadly, with no mechanism to transport food, this is just a pipe dream - which makes stuff like the Food Distribution Center utterly worthless - any world that would benefit from a FDC more than another farm has not even the slightest need for either one.