Tank size 6.5g - established approx 6 weeks
filtration - charcoal/sponge driven by air pump
ammonia 0, nitrItes 0, nitrAte barely registering after heaps of water changes.
pH 7.5
pH 7.2 out of tap
temp 25C

This tank had been vacant for a couple of weeks so I did a 50% water change before adding new fish. Readings before water change were ammonia 0, nitrItes 0, nitrAtes 20, pH 7.5.

Yesterday we bought 4 platies and a golden algae eater. They were acclimatised and moved into the tank well. They appeared to be in good health and good spirits, no signs of stress. They didn't seem familiar with the tropical flakes so I gave them some betta bites (crumbles) and they gobbled them up.

This morning one is dead and another 2 are very weak. The dead one was upside down on the bottom. One is floating upside down at the top although occasionally it swims around normally. Another is resting on the bottom, occasionally little swims around the bottom. The 4th (and largest) seems to be doing fine.

The tests this morning revealed nothing but I did a 30% water change anyway. There must have been some pH buffer still in the water (from the previous inhabitants - plecos) because it still read 7.5. An hour later I did another 30% change, salted to .1% and changed the sponge cartridge (in case some nasties were leaching back into the water). pH is still 7.5, that is weird, I have done 1 x 50% and 2 x30% changes but the pH is holding.

I understand that platies prefer 6.8 - 7.2, I know 7.5 is a little high but surely not enough to kill them. If the pH was a problem wouldn't they have acted stressed from the moment they moved into the tank?

I am beginning to think that I overfeed the betta crumbles and the fish are sick and dying due to overeating, is this a possibility?

Hmm... I personally don't think the pH is high enough to cause that sort of reaction. My platies' pH is 7.8 or 7.9, and they're happy as a pair of clams. All the dead platies I've experienced (and there were a handful...I bought sick ones thinking I could save them) happened before I moved them to this metro area, which has the higher pH. Well, with the exception of one, who was just too small to overcome the filter intake current.

Overeating is a possibility, I guess... they are quite small. I'd imagine it'd be relatively easy to overfeed them. What'd you do with the flakes after they went uneaten? Did they eat those too?

Sorry bout your losses. I hope you figure out what's making them sick.

Gee, I wish I knew more about Platies... but I don't. I hope you get this figured out soon! I will try to do some reading. That is all a very odd picture, though!

Thanks Kortniee, I feel better knowing that they can tolerate higher pH levels. My pH would only be .2 - .5 higher than the lfs because we use the same water supply. So, considering they were acclimatised over a couple of hours to the new water I don't think it is pH shock.

The flakes sunk to the bottom, some of the fish were eating them, but some didn't like them, were spitting them out. I only added a small amount so the sunken flakes may have been eaten later. I will keep an eye on water params but I need to get a smaller gravel vac for this tank. My large ones drains 3/4 of the tank to do a vac.

One more thing, we were going to get guppies but they had ich, so did another tank of neon tetras. I can't see any visible symptoms of anything but perhaps they have some disease. Who knows?

edit: thanks Graham, we were posting simultaneously.

Went back to the lfs this afternoon to check on the platies - all healthy.

I lost the golden algae eater and another platy during the day. This leaves one healthy one and another on it's last fins.

I am now of the mind that something is toxic in that little tank even though the water parameters are fine. Earlier today I changed the filter cartridge and now I have stripped down the tank and removed the gravel, as it may have been harbouring deadly bacteria or toxic waste such as hydrogen sulphide. It is essentially a new set-up now with no biobugs so I will need to cycle again.

I don't expect the sick one to survive but I hope the healthy hasn't been compromised by whatever it was.