HELP!! what's happening in my tank

[razman]

Tank - Marineland 29 gallon
LR - 20 lbs (Fiji/Brazilian)
Fish - Bicolor angel, tomato clown, 3 damsels and 1 yellow chromis
water parameters - nitrate - 10 ppm, nitrite - less than 0.2 ppm, ph - 8.0, ammonia - almost 0

2 weeks back I changed substrate from crushed coral to live sand +play sand combo (20 lbs + 50 lbs respectively).Prior to the change, moved my fishes to a 10 gallon tank with the cycled water. Since the water became cloudy almost milky, I did a major water change (tap water, dechlorinated it and set specific gravity to 1.023) and let the tank run for 2 days after which I introduced my fishes and the cycled water.

Once the water settled, I introduced around 6 lbs LR.

The water parameters measured have been taken today.

THis is what has happened -

Diatoms appeared cause all the rocks were covered with brown dust like stuff.
All my fish have died within the last 5 days.

I have been conducting water quality tests throughout the last 2 weeks to make sure everything is ok and levels are really not high. This has always been the case.

Please help, I do not know whats happenin here?

 

[steve-s]

Sorry for the losses

Depending if the LR was not very well cured, it could casue a substantial ammonia spike in the water which could be the cause of the fish deaths. Also, using tank water does nothing for colonizing bacteria. The main benefit to using tank water in a QT or temp tank is to match salinity, ph and temperature making it much easier and safer to transfer the fish.

Whenever a major change like the substrate is done, you are best off not doing/adding anything new to the tank for sometime after to allow it to become stable again. Substrate swaps are about as major a change as you can get and need time for the proper nitrifying bacteria to colonize the new sand.

Before restocking, make sure your NH3 and NO2 levels are zero. Then it will be safe to start adding new animals. If you are planning any more LR additions, now would be the best time while a new fish is being quarantined prior to introduction to the main tank.

Cheers
Steve