New Death Tank

[TookyFL]

Here's the background information:
I've got a new aquarium that I setup on 10/4. It's got Caribsea Eco-Complete substrate (80 lbs) and Seiryu Stone (~45lbs) for decorations. Filled the tank up with tap water (Fairfax, VA) and treated it with Tetra AquaSafe Plus Aquarium Dechlorinator Water Conditioner.  On 10/6, I added Tetra SafeStart Aquarium Cycling Water Conditioner to cycle the aquarium. I let it run until 10/9 when I heavily planted the tank. I tested the water conditions almost daily. I saw a small amount of Ammonia (0.25), never any nitrites, and always some nitrates. My water tests from today are below. I am running a Fluval 406 with stock media and currently utilizing a Tetra Whisper Air Pump (AP150) with the air flow dialed way back with one small air stone. Because of the plants, I'm using an AquaticLife 48 Inch 4x54 Watt T5 HO Light Fixture. It's on timers and runs 12 hours per day.
 
Aquarium/Water Details:

• Tank size: 75g (48L x 15W x 18D)
• pH: 7.8 (API Master Kit)
• ammonia: 0 (API Master Kit)
• nitrite: 0 (API Master Kit)
• nitrate: 10-20 (API Master Kit)
• kH: 120 (Tetra test strips)
• gH: 150 (Tetra)
• tank temp: 76 degrees

 
Plants:

• Anubias Barteri - 4
• Anubias Frazeri - 4
• Java Fern - 5
• Micro Sword - 10
• Jungle Val - 15
• Narrow Leaf Ludwigia - 4
• Moss Ball - 1
• Green Myrio - 3

On 10/10 I added my first six fish (Zebra Danios) to the aquarium. I floated the bag for about 21 minutes and then added the fish (not the pet store water) to the aquarium. Three of them appeared to take very well almost immediately... swimming around together. The other three seemed to hang out in the top corner of the tank out of the way of the filter output current or air bubbles. About four hours after adding the fish, two of them suddenly died. Thinking they died from the stress of transportation, I decided to add some additional since it is a large tank and they are schooling fish, so on 10/11, I went back to the same major-chain pet store and purchased six more to bring the total up to ten. Today, I'm back down to six fish. In total, six died -- 50%.
 
I'm not quite sure what's going on. I know my pH is a bit higher than what the textbook says Zebra Danios like. Because I was concerned about this, I asked the LPS what pH their aquarium water is and the kid salesman without hesitation said 7.2. I, of course, went home and tested their water and it is also 7.8, which is the same as mine and the same that comes out of the tap here in Fairfax county.
 
The fish that die seem to hang out at the top corner of the tank (without current/bubbles). I've noticed two of them gulping at the surface of the water like they don't have enough oxygen. Ironically, in the first batch of fish that I bought, I got a tiny baby fish. Aside from being a bit of a loner, he appears to be fine. You'd think if there was a problem with my water, he'd be the first to die since he's so small.
 
I'm confused and don't want to add any more fish to this new "death tank" until I figure out what's going on. What do you think is wrong?
 
Thanks very much!
Rich

 

[techen]

Whats the surface movement like, loads of ripples etc? It sounds like ammonia or lack of oxygen or rather both

 

[DreamertK]

What are the test results now for NH3/NO2/NO3? Like right now, not last time you tested.
 
The calculator says your tank is only 56 gallons too, at 48"x15"x18".

 

[attibones]

How much ammonia did you add while you cycled, TSS sometimes works but not always. If you never added an ammonia source, you never cycled your tank. If you get some fast-growing plants like frogbit, duckweed or other floaters, they will help keep your stats within range.

 

[TookyFL]

Whats the surface movement like, loads of ripples etc? It sounds like ammonia or lack of oxygen or rather both

Surface movement is fair. I backed the filter flow down a bit because I thought the tank had too much current for the fish. I've also got an air pump that's causing some additional surface agitation.

What are the test results now for NH3/NO2/NO3? Like right now, not last time you tested.
 
The calculator says your tank is only 56 gallons too, at 48"x15"x18".

I will test again tomorrow and post results. My type-o on the tank dimensions. It's 48" L x 15" W x 24" D; it's 75 gallon.

How much ammonia did you add while you cycled, TSS sometimes works but not always. If you never added an ammonia source, you never cycled your tank. If you get some fast-growing plants like frogbit, duckweed or other floaters, they will help keep your stats within range.

I didn't add any pure ammonia. I added TSS, ran for about 4 days, then added six Zebra Danios thinking they would be my not-too-much ammonia source.

 

[attibones]

There is your issue. Without an ammonia source, your bacteria likely never developed. You are now fish-in cycling. The best thing you can do is rehome the danios and start a cycle with ammonia. Your plants will be fine through this. You could also do a "silent cycle" by getting some faster growing plants (like the ones I mentioned previously), these will help keep your toxins low and provide cover and foraging space for your fish. In a silent cycle you only add a few fish at a time to be sure the bacteria load can take it. It's up to you, but remember that even small amounts of nitrite and ammonia can cause long-term damage to fish. On the plus side, six smallish fish in a 55 gallon won't pollute the water as quickly as a six inch Oscar and an eight inch pleco (neither of which are suitable for this size tank, but this is just to give you an idea).

EDIT: you said 75 gallons, I typed 55, same stuff applies though.

 

[TookyFL]

When I originally posted this thread, I forgot to mention my water was cloudy.
 
Update for today... The water is beginning to clear up quite a bit. I retested the water with an API Freshwater Master Test Kit.

pH: 7.8
ammonia: 0.25 ppm
nitrite: 0 ppm
nitrate: 10-20 ppm
tank temp: 75 degrees