Dead fish..... so confused.

Question
So here it is,
I have a 45 gallon tank, Its been established now for 6+ months. (Btw this is my first tank) I recently have had alot of guppie deaths in this tank. The basic make up of the tank was (is) 6 fancy guppies,  6 sunrise guppies, 4 neon tetra and one loner see thru ghost lookin guppie (inherited it). all males.

My tank levels are
Temp- 78
Ammonia - 0 to .25 ppm
Nitrite - 0
Nitrate - 10 - 20 ppm
Ph - 7.4

For a filter I have an eheim 40-70 gallon canister. ohh and I have 2 air stone bubbler display things.

My problem is, Ive lost somewhere around a dozen+ guppies in the past 3 weeks. They were all just fine till I bought a beta and stuck him in the mix on the advice of a friend who said they would thrive in a community tank.... Well I think it was the beta, day after day i would find at least 1 dead on the bottom of the tank with his back fin riped apart.

I took out the beta 4 days ago and had one die that night, But he had no damage to his body.

Since then I havent had anymore die but now they are all acting really weird. Alot of the time they swim like normal, but there are a few sunrise and the one ghost looking guppie who lay at the bottom or under the decorations, and dont barely move... Most of the time Im trying to net them thinking they are dead when they move.

When they do move they dart very very fast, then settle back down. Also they seem to be breathing very fast. More so than usual. I had this happen when i first did the tank to a few and they ended up dying soon afterwards.

The only other reading I can think of is the water is a little hard. Any idea if its anything or am i crazy?

Answer
Hi Nicholas

Well, Bettas should never be put in a tank with Neons and Guppies. I think you're "ghost" Guppy is actually a Glass Fish, but not certain. It's very fortunate that the entire tank wasn't killed. Bettas can mix in community tanks, but there are a few guidelines to follow. One, avoid fancy fin fish like Angels and Guppies. Two, avoid bright colors like Neons and Cardinals. Finally, avoid aggressive fin nippers like Barbs and most Tetras. Guppies and Neons should always be kept with peaceful fish of similar size.

You need to get your ammonia and nitrates down with water changes. All your levels should read 0 in a 6 month old tank. Replace the carbon in your filter and do 25% weekly water changes and make sure you are cleaning the gravel thoroughly with a gravelvac.

I also think the fish behavior you describe is actually these fish hiding from the Betta, not realizing he is gone. Try rearranging the decorations. I do this occasionally in my aggressive Cichlid tanks to minimize aggression. Sometimes it works, sometimes not.

Hope this helps, good luck!

Richard