On the fourth day, I woke up early and saw them all resting on the bottom of the tank, as DP's do. However when I came home that afternoon, I found Theo upside down on the sand, dead. He was eating and moving a lot the day before, his color was good and he seemed very active. The next morning, I checked my tank and Ben was not moving around as much as usual. I assumed this was because him and Theo always swam together, maybe he was shaken up. I then took dead Theo to the store and they couldn't figure out why he might of died either. They tested my water and told me my ph was a little bit higher than they kept them at the store. (I read DP's live in 7.2-8.2 ph, my ph was at a 7.6, but their ph was kept at a 7.0). So I bought the powder for my tank to lower its ph, and I bought a new DP, since I prefer to have them in groups of 3.
The new DP I bought, I named George. He was older because he was distinctly male with his black stripe going across his belly. Before I put him in the tank,I noticed Ben was still lethargic. Ben had eaten the night before but ignored a snail I'd given him that morning. I slowly introduced George to his new tank, and immediately Leo began to chase and nip at George (even though George is twice Leo's size!). That's how I came to my conclusion that Leo was also a male, since males are very territorial, and that Ben was female, because his (her) body was more rounded, and did he/she did not mind the other two fish at all. I left for the night after introducing them and came back late tonight to find Ben dead, in the bottom of the tank. What?! I don't understand why they keep dying! Please help!
Q: Does the current of my filter effect this at all?
1) Water parameters -
pH - 7.6, Ammonia- 0, Nitrite- 0
2) Tank size and a list of ALL inhabitants -
10 GALLON with 3 DP (now 2

3) Feeding, water change schedule and a list of all products you are using or have added to the tank -
- food: 3-4 live blackworms per DP once a day, along with accessible ramshorn snails always in tank
- products: stress coat (when water change), Water conditioner (when water change), Discus buffer (1tsp daily till ph gets to be 7.0)
- water change schedule: I have not changed water yet because has been less than a week since tan was set up, but I had planned to alternate between partial water changes and full water changes weekly. (I will change the water tomorrow)
4) What changes you've made in the tank in the last week or so.
On the 3rd day of having the tank, I added a rinsed aquarium castle from my boyfriend's tank to the middle of my tank to give the fish more places to hide. Other than that, everything else has been constant.
5) How long the aquarium has been set up, and how did you cycle it?
This aquarium has been set up for 6 days now (as of 12/22/18), I cycled the tank by letting it sit for two hours till the bubbles were settled, with the fish (still in the bags from the store) floating on top.
Why are my DP dying? PLEASE HELP!