I Just Killed My Sons Betta

Help I have lost 3 Fish in the last 3 days. Just an hour ago my 4 year old's betta that we have had for 3 months died. I am almost sure that I brought home a disease from the LFS when I added 6 Endler/Guppy hybrids 4 days ago. The endlers were drip acclimated while I removed the betta from the tank and rearranged everything. I then added the endlers first and after and hour reintroduce the betta. After a few minutes of flaring and chasing the betta seemed to settle down and accept his new roommates. The tank had recently finished cycling and adding these 6 little fish started a mini cycle. The next morning I tested the water and had .5ppm ammonia and a trace of nitrite. Did a 90% water change and checked again that night. This time had .25ppm ammonia and .25ppm nitrite. Did another 90% water change. 2 days ago the morning test showed only trace ammonia and nitrite. That evening I checked again, 0 ammonia .5ppm nitrite so another big water change. Also 2 days ago I couldn't find one of the endlers and just assumed that he was hiding. 2 of them didn't like to socialize with the others for some reason anyway. 4 would be hanging out together and the other 2 would be off doing their own thing not caring what else was going on in the tank. Yesterday morning I found the missing one dead under some decoration while doing a water change because of another .25ppm nitrite reading. This morning only trace of nitrites but I found another endler just laying on the bottom. I went to get him out and he shot across the tank. I noticed that he had a chunk of his tail fin missing. Not sure if it was like that before but it was the only thing I could see wrong with him. Couple of hours later I found him dead in the gravel. Then I noticed my sons betta acting sluggish. Another check of the water showed .25 nitrite so changed the water. Watched him real close for 30 minutes and he would fall to the bottom totally lifeless then shoot across the tank like he was running for his life banging into the side of the tank. Then would be pushed around by the current until he landed lifeless on the bottom again. He did this every minute or 2 for 30 minutes. I pulled him out and put him in a small cup full of tank water so I could watch him closer and within a minute he was dead. Any ideas what is killing my fish and how to save the other 4?
Tank size:10 gallon
pH: 8.0
ammonia:0
nitrite:0
nitrate:20
kH:unknown but we have hard water here
gH:unknown but we have hard water here
tank temp:78F

Fish Symptoms (include full description including lesion, color, location, fish behavior):Color unchanged from normal, laying on the bottom with sudden burst (2-3 seconds)of energy then acting totally dead just pushed around by the current and then laying on the bottom for 30-45 seconds till the burst of energy again

Volume and Frequency of water changesaily 80-90% water changes (currently in a mini cycle)

Chemical Additives or Media in your tank: Seachem Prime

Tank inhabitants: Adult Male Betta (been there since we got the tank, just died an hour ago)
6 adolescent Endler/Guppy Hybrids (2 recently deceased)

Recent additions to your tank (living or decoration):3 plants 2 small pieces of bogwood and the 6 adolescent Endler/Guppy hybrids all from the same LFS added 4 days ago

Exposure to chemicals: none known (dechlorinator is the only thing I add to the water)

The problem with a 10 gallon tank is things can go very bad very quickly. Your tank can only support about 10 inches of adult sized fish. The 4 guppies you have are about all you can have. If they're not all males you will have more. A bigger tank gives you more time to prevent disaster, more water to dilute the ammonia and nitrite that kills fish and more room for fish. If you want to keep your present tank or until you can upgrade this is what I would do. You're in the US so you can buy an active sponge filter from
http/www.angelsplu...ltersSponge.htm. Have it shipped as fast as possible. Until it arrives you're gonna have to test your water with a liquid test kit at least twice a day or more and anytime you get any trace of ammonia or nitrate you need to do as large a water change as possible (leave enough water for your fish to swim upright). Refill with temp matched water and Prime. When your sponge filter arrives take all the media out of your present filter and put the sponge in, you can cut it up if you have to to make it fit. That sponge filter is loaded with the good bacteria you need for an almost instantly cycled filter. Keep a close eye on your water parameters for a couple of days after the sponge is in. Do weekly 30-50% water changes and gravel vacs.

Just lost another fish last night. The Cycle isn't the problem. Checked stats just a minute ago. Only a trace of nitrite and no ammonia. Clearly these guys are carrying some disease that is killing them but I have no idea what. They seem to go from completely healthy to laying dead on the bottom in less than an hour with only a few minutes of the odd behavior described above.

Even though your water is ok now, the earlier water conditions may have damaged the fish.

Well we lost one more last night and one died this morning while I was changing the water. Only one fish left in the tank now. If he survives he will be the only one. Has no one heard of a disease that does this? I just can't believe that a small ammonia and a couple of little nitrite spikes is killing the fish like this. I am using Prime so it should be detoxifying them for the few hours until the water change.