Usually I follow deaths with a change in the tank, or just leave an empty space, but in this case I feel so guilty and terrible that I really want to get another one right away. Two people have already stepped up to help me - I may be able to get a 3-4" guy, plus my girlfriend asked the local store to grab one from the wholesaler (though that one would be 2-3" and probably 70 bucks. Might be too small for my tank). Part of me feels like it's insensitive to the two that died to replace them so quickly but most of me is just eager to get another smiling bright red fish. One will do instead of two this time, though.
I would suggest that your experience is possibly a combination of the following:
-Too many fish (too much fish mass)
-Poor circulation
-Elevated temperature
While part of me wants more responses suggesting other possible causes so that I don't feel so guilty, I'm virtually positive it's the circulation, first and foremost. Especially since the Lussoso stayed in the tank the whole time and recovered. And my AT is back in there too with him now, since he ate at least two tetras and made my girlfriend cry.
I might be pushing the limits with the bioload (18 fish, 2 of which were 9"), but I have always had good success with overcrowding. I'm just so upset because I could have waited a week to remove the HOB backup filter, until my FX5 arrived. Because if that was there there never would've been a problem. Sure the Eheim could handle the bioload, but putting the HOB on the planted tank was by no means an emergency or necessary. In fact so far I haven't seen any difference.
Last summer I had overheating issues, even with the tank in my place's bottom floor, which is basically a basement. This year all my tanks are at my girlfriends, which has a solid air conditioner, so I'm hoping to avoid any issues like that. It's a steady 80-82. I lived in Fremont for a short time long ago and I remember a streak of two straight weeks of 100+ temperatures one May (01 I believe). I can't imagine how difficult something like that would be on a garage tank.