Fish die-off but a "new" tank

[PostalPenguin]

So I haven't been on here in awhile, have been busy with life and work. But my aquariums were plugging along and all the fish were happy. However, my 29g was over-run with algae. I just couldn't seem to get it under control. Combined with the fact that I will be moving in a few months made me decide to consolidate my 20g and 29g into the 29g while nuking the 29g of all algae. So the final stocking would be: 6 tiger barbs, 2 rams, 5 platys, 7 black neon tetras, 2 otos and 6 juli cory. A bit over-stocked, but I figured since it was heavily planted and over-filtered I'd be fine.

I first had to remove all the fish to a bucket so I could clean and nuke(aka bleach) the tank. My plan was to preserve the bio-filters by simply just not turning them on while I was bleaching the tank. Since I was only bleaching the substrate I figured this would be safe. I did several water changes until I could no longer smell the chlorine and used excess dechlorinator at each step.

Well once the tank was cleaned and replanted I added the fish and went to bed. By next morning the death toll was 4 tiger barbs and one ram. The rest of the fish were gasping at the surface. I immediately thought that bleach was still present so I added more dechlorinator. Water tests showed no ammonia or nitrites. To be safe I added used bio-media from the canister that was on my 20g. By the afternoon, nitrites were showing up so I purchased Prime and began dosing. Fish seemed completely recovered within a few hours of the Prime dose.

Today I lost the other ram. I checked the parameters and I'm going through a mini-cycle with ammonia at 0.25 and nitrites at 0.5ppm. I do not know what caused this. Perhaps enough bleach remained to wipe out the bio-filter or the addition of the fish over-strained it. Maybe a combination of both. Filtration, by the way, is an Eheim 2213 + Eheim Ecco 2236. All I can do now is wait and hope the established media is enough to kick start the tank. Once the plants get going they will probably cycle tank very quickly too.

So the current stock is now: 2 tiger barbs, 5 platys, 7 black neons, 6 juli cory, and otos. I am going to get 4 more tigers and probably one or two more platys.

But anyways here is the tank:

[Lab_Rat]

Combining the bioload of two tanks and bleaching out any biofilter in your substrate was probably too much for the remaining biofilter to handle. I would not have made such drastic changes at one time and I also wonder if you had any residual bleach, even though you rinsed and extra dechlorinated. I'd let your tank stabilize for a couple of weeks before adding any new stock. Sucks that that happened.

[Dixie]

I don't know what happened, sorry about the loss though, but I agree that waiting a couple weeks to add more fish would be a good idea. Your tank looks great by the way. Love the plants.