Bettas, Platys and guppies

Question
QUESTION: Hello, Jaymie:  I have more than one question.  On the advice of some very "helpful" store associates, I bought a male and female Betta.  I have no interest in breeding.  The male is in a 2.5 gallon tank and the female a 1.5 gallon, both with undergravel filter and heater.  I now realize that the tanks are too small.  Four weeks ago, I bought the largest tank I have room for, a 20 gallon long.  I set it up with a power filter, regular store gravel, artificial plants and decorations.  I added a small amount of gravel from both Betta tanks (they're 6-8 months old-water changes between 1-2 weeks each), bacterial supplement, water conditioner and heater set at 76F.  After 5 days I added 3 Platys which were supposed to be male.  A week later, I added 4 guppies.  All guppies died within 2 weeks with no apparent illness.  (all came from the same store)  Now, one of my Platys, which I suspect is female, is being chased by a definite male, I expect trying to mate.  I feel it is too soon to add other Platys to relieve "her" distress.  Tonight, she appears frantic and is constantly hitting the glass.  She is eating.  I do water changes, between 15% and 50%, about every three or four days.  The 50% was after each guppy died.  I replaced my filter medium 3 days ago after an ammonia spike to .05 otherwise all water levels have always been fine.  I test everything daily.  What is wrong with her and what do you suggest I do?
Back to the Bettas.  Can I move one to the 20 gallon tank, and if so, which one?  Both appear very healthy so I'm concerned about the move.  Would the other be all right in the 2.5 gallon tank?  I'm out of room otherwise.
I hope I have not been overly wordy but I wanted to give as much info as possible.  Thanks.

ANSWER: Tricia,

 I will try to answer in order.

1. 2.5 gallon tanks are the minimum size for a Betta. Under-gravel filters are not only unnecessary but I do not use them an any of my tanks.

2. it sounds to me that the 20 gallon tank was not cycled. This is an absolute must when keeping fish. The cycle takes 2 to 3 weeks and is done with just the water, filter and heater. No fish. This sets up the beneficial bacteria in the water.

3. When adding fish to a cycled tank, it is always a good idea to quarantine the fish for 2 weeks before adding them to the tank. During this time you are making sure the fish are healthy. If there is anything wrong with the fish, you don't want their illness to be spred to the other fish. If after the 2 weeks your fish is doing well, then you can add them to the main tank.

4. never add bacterial supplement to your tank.

5. Chances are the guppies were sick when you bought them. Unfortunately I do not understand what this means "(they're 6-8 months old-water changes between 1-2 weeks each)" so I cannot help you with this one.

6. When doing water changes, it is 25% done on a weekly basis. Every 3 or 4 days can be too much and send the fish into shock.

7. for now I would remove either her or the male. If she keeps hitting the tank, she is definitely stressed and that can kill her. You can either remove her or put up a partition in the tank. If you do it that way, she can stay in the tank but will be separated from the male.

8. The male Betta's are very content on being by themselves. If you are going to add male guppies to the aquarium, then putting the male Betta in is not a good idea. The female however should do quite well with tank mates.

I hope I have helped a bit and if you need any more help just let me know.

---------- FOLLOW-UP ----------

QUESTION: Thanks for the very rapid response.  As far as your answers, please understand that as a new aquarist, I have followed the directions of several books, websites and store personnel on how to set up the aquarium and cycle it.  These sources stated to add hardy fish in small amounts and add bacterial supplement to cycle quickly.  Honestly, now I feel horrible in how I've done this.  I was doing the frequent water changes to lower the ammonia.  What can I do to lower it?  Also, since the tank is 4 weeks old, can I add a couple more Platys?  Other than the female Betta, I think I will stick with Platys for now.  Oh, should I get another female Betta for her to have company?  Thanks for all your help.  It is much appreciated by myself and granddaughters.

Answer
Tricia,

 Yea, unfortunately pet store personnel do not know too much about fish. A fish-less cycle works better than a fish cycle and here is why. If you have fish in the tank while cycling, it will take longer because you will be adding ammonia to the tank in the form of waste. If there are no fish, you bypass that problem. Having the fish in the tank while cycling can be done, but it takes longer. You are right. The fastest and easiest way to remove the ammonia is by water changes. Test the water everyday, and if the ammonia and nitrites are high then do another 25% water change. Depending on the water conditioner you are using, you can add more of it and it will still be safe for the fish. It should say how much on the bottle. I would not add any more fish until the ammonia is gone. If you add fish to an ammonia filled tank, they will die. A good way to change the filter is to put the replacement in the tank water for a couple of days before hooking it up. This will give the filter time to get the good bacteria from your tank. I understand that starting out with a new fish hobby can be difficult. I would suggest finding just one book or website to follow. There are so many of them and all will say different things and that can become very confusing. I have had and aquarium for 30 years and I am still learning things. As for adding another female Betta, that would be a no. If the females are from the same fry batch and have grown up together then yes its fine. They are just like the male and will fight until the death. One female in the tank will do just fine. Any more question, feel free to ask. Good luck with your tank.