Sheesh will everyone just lighten up, B) be cool all
A - I have heard of 10-40% unconditioned water changes killing fish, this is generally after large rains the local water companies add more clorine/cloramine etc to kill any stirred up bacteria, also depends on whats normal "treatment" for local conditions, on the otherside I know people that do 10% weekly changes without water treatment, just remember that a system used to conditionedwater will have more problems with a sudden input of unconditioned water than a tank always run with unconditioned using mor unconditioned
B - Dead/dyding plants may of killed your fish by giving of ammonia/nitrite/nitrate at higher levels than nornal more so if the tank was used to using these plants to absorbs the toxins.
C - could of been A + B
D - could of been someone spraying chemicals in the same room as the tank
That sucks! But, 1 20% water change without conditioner shouldn't kill all your fish.
breeze7050, what makes you say 20% water change?, I do 50% 7-10 days
Sand in the tank, while personally I have never experienced it, I have heard it is possibleto have toxic gas build up in the sand if not regular combed/disturbed, just a possibilty, but I doubt it.
If you weren't able to do tetras/gupies/corys successfully I don't think you are ready for dwarf cichlids, these are best left to people who have more experience.
I've had more trouble with neons and guppies than I ever did with dwarfs and I've been keeping both, and leaning towards dwarfs (including breeding) for over two years
I'm not sure what foot print a 105l has but...
apistos in general need 2 square feet for a trio (1m/2f) a pair of rams need 1 square foot so you should be able to get a tro of apistos and a pair of rams with maybe a modification of stocking later down the line (6-10weeks)
tank mates - panda or other small cory types go well as they dont get to big for the dwarf to keep away if they breed, add 2 bristlenose catfish and some ditherfish eg small tetras, guppies, sparkling gourami and maybe a dwarf gourami or two, wait a month or two see how the tank goes modify as needed
Basically I'd reset the whole tank, wash the sand, wash the plants (if any left), wash the filter and heater, refill let it settle, clone filter media, add fish
you might want to consider 10% water changes every other day for the first week if your paranoid
The main thing to watch for the quality of the stock you get and being careful when you add the fish/acclimitise
HTH
Andrew