So The Cycle Is Finally Done

[rotex5510]

4 tanks:
2.5 gallon - Betta - recovered well from bout of fin rot
10 gallon - 8 platys and 1 danio, waiting out their month of quarantine following ich (I've had most of these a few months, they survived in the fish in cycle tank)
26 gallon - 3 dwarf platys, 3 harlequin rasbora, 6 zebra danio, 7 guppies, 2 or 3 guppy fry (possible), 1 rubbernose plec - at beginning of treatment for ich outbreak number 2 - fishless cycle completed 3 weeks ago

and the problem tank
55 gallon, pool filter sand, live (compacta, java fern, java moss, aponoganton? and amazon sword) and plastic plants
completed cycle last week, fish added 5 days later after 90% water change, on Wednesday
All fish were recent arrivals (same day) at store
Fish acclimated in bags in tank with 1 cup tank water added every 15 minutes for 1:15
6 peppered cory
6 julii (probably trillineatus) cory
2 brochis splendens
2 female play (1 dropped fry within 24 hours, most of which seem to still be swimming in the tank)
1 male platy
1 rubbernose plec, added saturday

Stats:
pH 7.4 or 7.6
temp 76 degrees
nitrites 0
nitrates 10
ammonia 0

additives:
API tap water conditioner and stress coat
API leaf zone (less than bottle recommends)

deaths through yesterday:
2 peppered cories (1 barely moved first night)
3 julii cories (2 trapped on powerhead, third was reddish underneath by dorsal fin and swam in a spiral towards the end)
1 female platy (had clamped fins from day 1 and would sit at back of tank)
1 male platy (began to exhibit same behavior after 3 days)
At least 2 of the remaining juliis exhibit some flicking of their heads into the sand once in a while. Otherwise, the remaining fish are very active with good appetites and no clamped fins.

It just seems like an awful lot to be losing after finishing a proper fishless cycle. I lost 4 corys back in the beginning (April) before I found this site, 2 danios and 2 platys during a cycle, and 2 male guppies due to aggression. Could this just be weak stock? A couple of the peppereds in the store tank didn't look so good. Everytihng I've bought except the rubbernoses and 3 dwarf platys have been from the same shop. What else could be going wrong?

 

[waterdrop]

Well, you're bringing in a large enough sample of fish that you could just be seeing some weak stock, it happens.

~~waterdrop~~

 

[OldMan47]

That does seem quite a bit to lose all at once Rotex. Most of the cories are gone or threatened by present symptoms and even 2 of 3 platies dead. I have never seen a platy just up and die after being introduced to a new tank.
Do you know whether or not you have hard water? A high mineral content will be hard on the cories if they were being kept, as my LFS does, in a mix of tap and RO. Cories do tend to be among the more vulnerable fish that we keep so soft water might even explain things better. Losing cories just because they are cories and losing the hardier platies to a low TDS. Many of the more common livebearers do not do too well in soft water but usually don't succumb nearly that fast. The 7.4 to 7.6 pH is promising for the livebearers but not if the water is too low in minerals. Clamped fins are a classic symptom of low TDS for a typical livebearer.
I would avoid feeding the plants anything until you can figure out what is happening to the fish. The plants will survive for weeks with nothing special added to the water although they won't grow quite as fast that way.