Thanks for your help.
I have a Fluval Roma 125L
My inhabitants are: 4 young platys, 1 molly, 2 glowlight danios, 1 spotted danio, 2 zebra danios, 5 neon tetras, 7 harlequins, 2 cockatoo dwarf cichlids, 1 pygmy cory, and 3 penguin tetras.
The 2 pieces of bogwood and this tank have been set up for around 3 months (although all the fish and water were transferred from a previous smaller tank).
I vacuum my tank every 1/2 weeks and clean the sponges every 2/3 months or so (I last did it 1 month ago).
My water stats are as of now: nitrate- 50mg/l
nitrite- 0
ammonia- 0
pH- 8.2 (I think this is as a result of that bottled water as you suggested)
I have had recurring disease-related problems for around 1/2 months now. I have a test kit that includes KH, GH, nitrites, pH and ammonia, and everything has been in balance. At first I had a guppy that died with what I now think is dropsy. I then has a danio die with swimbladder. I had a platy that also died with dropsy. My chinese algae eater died mysteriously.
All these problems have come about all of a sudden, so I took a water sample to my LFS and they said that the nitrates were very high (80mg/l) so I bought a nitrate test kit. I put a pot of live bacteria that they sold me in my tank, but it still read between 50-100mg/l in my tank. I also treated with NitrateMinus however nothing changed. I then did a 20% water change with bottled water that I tested to have 0 nitrates in; but still nothing has changed.
Now I am not sure if one of my neon tetras has dropsy! Please help!
I am going to do a water change now to try and help the stats. I will change around 40% with tap water (I did a very good vaccum the day before last) and add AquaSafe, change 1 of my filter pads for new ones, and add some aquarium salt. Fingers crossed this will work, but if you don't think I should do any of this- please say! Thanks!