Platies Dying Suddenly

Can anyone help me figure out why my platies are dying for no apparent reason? I had 7 platies and a pleco in a 75 gallon tank. I have had the tank set up for 2 and a half months and it has been cycled for a month now. 2 days ago I found 1 platy floating on the top completely drained of color, and this morning I found another the same way. Water parameters are 0 ammonia, 0 nitrite and 5 nitrate, I haven't added or removed anything from the tank since I started it and all 7 platies have been in from the beginning. Any idea what could have happened?

 

Have you tested the pH?

Probably best to sit and observe the remaining fish, see if there is anything they are doing that is out of the ordinary. Fish behaving differently to normal is a sign something is wrong, and depending on what they are doing may help someone to make a suggestion as to what may be wrong.

Tony.

 

Yes PH is 8.2, a little high but has been consistent since the beginning. I sit and watch them all the time lol that's actually one of my favorite ways to relax. They were both acting fine until about an hour before they died, however both were very slI'm compared to the 5 fat ones that look like they swallowed a marble or something ( females possibly pregnant??) The hour before they died they just sat listless at the bottom of the tank.

 

It could be something as simple as age. It could also have to do with fluctuating temperature, outside things getting in the tank like a chemical or something. Has there been any big changes lately?

 

Water temp is consistently 79° No changes since the tank was set up. I don't keep any chemicals near the tank including air fresheners etc. I clean the outer glass with a vinegar water solution that I make myself, I haven't had to clean inside as the pleco takes care of all that. I have only had the fish a little over 2 months but I suppose they could have been older when I got them from my lfs. I basically want to make sure there isn't something wrong before I start adding more fish.

 

If you're levels are all consistent, where they need to be and you don't see any visible signs of illness then I'd probably keep an eye on things a week or so then go ahead and add more fish.

 

K thanks for the help.