Need Help Starting New Livebearers/poecilidae Tank!

[FWFishLover]

I Am Going To Start A Community Aquarium With Poecilidae/Livebearers. But I Only Plan On Buying Mollies Swordtails Platies And Fancy Guppies.
I Have A 20g Tank (Which Is Perfect Because All These Fish Require 10g Minimum) I Already Have 25 LBS Of Gravel And A Big Hollow Submarine For Hiding As Well As A Plant (Type Unknown)
And A Mediumish/Smaller Fake Rock Which They Can Also Go Inside Of.
As Far As Fish Care Goes I Already Have A Filter And Heater (Tell Me If I Need Anything Else Please).
Also What Is A Good Algae Eating Fish To Mix With These Fish? I Was Thinking 1-3 Plecos (Tell Me A Good Amount Please)
I Already Have A Male Neon Swordtail (Clyde) And Am Worried He Will Feel Very Territorial At First.
Basically I Would Like Some Info On What I Need (That I Dont Already Have)And How To Prepare.

 

[OldMan47]

What I see that you need right now is to get that filter of yours cycled. When you first set up a tank and put the filter on it, the filter is only capable of removing particles in the water. The main function that a well established filter does for us is called biofiltration. When fish pass water through their gills they do several things. They absorb oxygen, release CO2 and release NH3. Unfortunately the ammonia, NH3, is poisonous to fish in small concentrations. This is where the biofilter function comes in. Here is where we get lucky in our tiny environments for our fish. Ammonia can be readily converted by bacteria to nitrites if the bacteria are present in large enough numbers. Unfortunately the nitrites are also poisonous to our fish in low concentrations. (You get the good news bad news thing going on here?) Fortunately there are other bacteria quite capable of converting nitrites to nitrates. Now here is the real good news, fish can tolerate relatively high concentrations of nitrates. We only allow them to build by about 20 ppm but that is much easier than the traces that are a problem with the other chemicals.
What we end up doing, as fish keepers, is allow the bacteria populations of both types to develop in our filters and then do our best to preserve the function of those bacteria. Meanwhile we need to protect the fish in the tank from the build of ammonia and nitrites that can happen with the fish breathing by passing water through their gills. Decaying organic matter can also add to the already bad ammonia problem but that should not be a concern at this point. We do that control by doing water changes whenever the ammonia or nitrite levels in a tank reach as much as 0.25 ppm. After a while, the filter's bacteria reach a level that both ammonia and nitrite levels settle at zero and the nitrates build up slowly. At that point we start doing routine smaller water changes every week, maybe only 25% each time. When you get to that stage we say the filter has cycled. A cycled filter is capable of handling whatever fish biological load you have at the time it first becomes established. At that point you start to slowly increase the biological load while monitoring the water chemistry. If everything stays good, you are going slowly enough on building your stock. This whole thing is a very basic description of fish-in cycling. Fishless cycling is very different. In a fishless cycle, we make the bacteria think there are tons of fish in the tank and let them grow their numbers while the fish stay at the local fish store. When the filter has cycled, we are fairly safe adding those large stocking numbers all at once in terms of the filter's ability to process the resulting ammonia.
When you have a cycled filter, please post in the common livebearers section and those of us who specialize in livebearers can help you a lot in making good choices of tank mates.