Platy Problem

Question
10 Gal community tank. Setup May 2006, finished cycle August 2006. Ran very high ammonia during cycle. Filter Fluval 3 submersible. Current conditions: Temp: 82F, pH: 6.8, Ammonia: 0, Nitrite 0, Nitrate: 30 ppm, KH: 1, Phosphate: 5 ppm.
Stock: 1 rasbora, 1 pained tetra, 2 glass cats, 1 female platy, 4 neon tetra, 6 cleaner shrimp.
Maintenance: Fish get fed every other day. Very light feedings, high variety. 30% deionized water change (with de-chlorinator) every 2 weeks, with gravel vacuum and freshwater rinse of half the plants and half the ornaments. I maintain pH and KH with baking power (1/8 teaspoon).

Tank suffered of chronic cloudiness (white). I experimented shutting off the lights, and some exterior fine micron filtration. I simply can not drop the phosphates to zero. Tried everything. It has dropped dramatically, it used to be off the chart (black). My tap water has 5 ppm phosphates. Even after I filter the tap water through DI, I can not drop it below 1 ppm. I addition to the phosphate heavy foods, and I tried some pH plus for a while (before I knew better) - The cloudiness was due to extra nutrients in the water. I also has a 5.5 gal betta tank that had green, smelly water.

Last week I purchased and ran a D1 diatom filter. The results, in both tanks, are stunning. Ran it for 30 minutes in the 5.5 and for 1 hour in the 10 gal. Both have crystal clear water, even a week later.

In the 10 gallon, I have lost 2 platys this week and the third one is hiding all the time. She is eating, but hiding. Neither of the other 2 platys looked sick of had stopped eating. They just seemed to drop dead. The fish are kind of lethargic in general. All are still eating. All the water tests checked out, but something is just not right. Is it a problem that I had only female platys? Could it be related to the diatom filter? Any other ideas as to what could be wrong? Thank you.  

Answer
Dear Lee,
Thank you for your very informative letter.
From your listed readings, it sounds like your aquarium is cycled indeed.

If your ammonia,nitrite, and nitrate all tested out to be OK. (ammonia-0 Nitrite-0 Nitrate- 20 or less) Then the mysterious loss of the platies needs much more looking into. The lethargic behavior and not eating are definitely not good signs. Make sure nothing foreign could have contaiminated your aquarium (air sprays ect...) But this is only a guess. Sometimes you can lose fish for some unknown reason. I really hope you do not have anymore losses. The best thing I can recommend is to do several 20% water changes on your 10gal to help straighten out any possible water quality issues making sure to gravel vacumm thoroughly. I say this small amount because a sudden large water change in a tank that doesn't get frequent water changes can shock the fish by the sudden addition of fresh water. Aquariums that have more frequent water changes can easily handle a large water change with no problem. The platies may have simply been weak to begin with or may been more suceptible to stress or an unknown illness. Above all, please do several large water changes (making sure that the replacement water is equal in temperature to that of your aquarium and making sure it is dechlorinated with a good water conditioner as well of course)

*It's also really best to do water changes at least once or twice a week rather than every 2 weeks. Buildup of pollutants and dissolved organics may be too high by the time a usual water change is due. I do partial water changes on all my aquariums 2-3 times a week and this works out extremely well.
*By usual standards, a nitrate reading of 30 is considered really too high.

Unless you have an unstable pH level due to not enough KH or Alkalinity, it's best to not try to try to adjust your pH since even minor flucuations can stress fish or make them very sick. It is much better to just leave your pH as it is and the fish will be healthier and better off for it. Luckily most all captive-bred fish are adaptable and hardy.
Dionized water is considered too 'pure' to keep fish healthy when used as a sole source. It lacks special minerals and other trace elements fish really need to be healthy and at their best. I would try to just use normal tap water if you can, if you still prefer to use the Dionized water, try mixing it with a little tap water to replace some of the vital minerals much needed by all your fish.

Sometimes fishkeepers may worry too much about their water conditions. Really the best thing you can do for your fish is keep the water clean with plenty of frequent water changes and the pH stable. Actually frequent water changes will keep your pH more stable and at a higher level due to the presence of very little decaying organic matter (which produce acid and therefore lower the pH)
pH can tend to lower overtime if water changes are not done frequently enough.

The green, smelly water you explained about your Betta tank is a serious sign that something is wrong. More frequent water changes and careful small feedings should help solve and prevent this from ever happening.

I really hope this helps! If you have anymore questions, feel free to write again...

Karen~