I have a 10 gallon planted cycled tank.

Ammonium: 0
NitrITE: 0
NitrATE: 0
PH: 7.5
TEMP: 78

3 otto, 5 nerite, 1 assassin snail, 2 RCS and 1 betta.

Six months ago, my betta was in a 1 gallon tank that was not cycled and did not receive proper maintenance. He has lost most of his color and has since then recovered from his fin rot. Let's call Bruce a "rescue betta".

He got transfered to this new tank as soon as it was cycled and ready for him. He did acted as if he was very happy. He swam and inspected very little corner and liked to come to the top front of the tank to say hi whenever I was around.
Last week, things changed. I introduced 6 otos purchased from Petland. 3 died in the first two days and I only found 2 of the bodies. I deemed the deaths a result of transportation and stress of a new tank (as per what I've read about otos, this is fairly common).

Since I introduced the otos, my betta has been acting sick. He hangs out at the very bottom of the tank and rarely comes up to say hi other than to eat. He barely moves anymore and seems limp. The otos, RCS and snails are very active and my tank gets a weekly 25% water change.

Other facts: I dose water sprite for the RCS (minimal but since then the shrimp have become very active, are a very bright red and do not even hide from my betta so they must be feeling good), I condition the water before water changes of course and I have fert tabs at the bottom for my plants.

Help! What's wrong? What can I do?