mysterious deaths...

:help:
I've got a 38 gallon tank with a dozen endlers, 6 dwarf rasbora, 2 cory habrosus, 2 aspidoras, 2 (male/female) albino cherry barbs, 1 bn, 5 dwarf pencilfish, 2 otos...
I've got some dwarf water lettuce and java moss and java ferns, that's it.

In the last 2 days I've lost 2 apisto. macmasteri, 3 aspies, 1 habrosus, 1 (last of 6) black darter tetra and an oto... no visible signs except I saw one aspidoras spinnning erratically 3 days ago but he leveled out...

My levels are 0/0/10 amm/ite/ate, I've got plenty of surface agitation and the ph is 7.2, temp is 80.

My latest addition to the tank would be some dwarf waterlettuce out of another one of my tanks, no problems in there though... other than that, the last fish I added was back in april and they were the aspies

Please help! I haven't had a die-off like this in 2 years, the last one was due to a co2 disasster, but I'm not running co2 anymore, haven't for a year

I have a mixture of silica and pool filter sand with mts to help it out a bit...

I live in western WA, but I doubt it's a problem with the main water line since none of my other tanks are having any problems.

The tank doesn't get anything special.

It's been 4 days since my last death, I wasn't here for them but apparently all was well, so I guess the rush is over, I'd still like to know what happened though