Things keep dying in my tank, help!!

[Deathwyrm]

I bought my tank 2 months ago. The tank is about 35L capacity, with OF water filter and air pump. The tank has a limited edition LOTR Lego set, gravel sand, and some stones (silicate based). Inside the tank I have a zebra crayfish, seven baby blue crayfishes, 3 yamato shrimps, and 20 tetras.

All went well for several weeks, with twice weekly 30% water changes. Then one night after work, I noticed that the water is cloudy, with slight white froth on the top of the water, and the fishes are behaving weirdly. I immediately initiated a 30% water change (the previous water change is just 1 day before), thinking everything will be fine. The following morning when I woke up, all the fishes died. The water has this weird chlorine / burnt cigarette butt smell, and I immediately did a 80% water change. When I came back home that evening, all the baby crayfishes died, and subsequently the zebra crayfish also succumbed to the condition.

Not knowing whats the condition, I cleaned the tank thoroughly, washed all parts that can be dismantled, soaked everything that can be dismantled into hot, near boiling water (including pump parts and stones). The water is totally changed and finally the whole tank is setup to its original configuration. However, several days after having nothing living in the tank, there is still some slight burnt / chlorine smell whose origins / exact smell cannot be traced. One week passed after the die out, I purchased some tetras, and 2 of my remaining crayfishes into the tank. The tetra seems fine on the first day, but are found to again behaving weirdly on the second day, now I promptly removed everything into spare tanks.

Some other background information:
Water used is from the tap, all water used are pre-treated with anti-chlorine and are negative to standard chlorine test kit.
pH of tank is measured with pH meter and averages around 7.8
Test kits for Nitrate, Nitrite, GH, KH, ammonia all turned up normal on every single tests done

The question is, whats wrong, and what can be done to turn around the situation?

[tetrakid]

I bought my tank 2 months ago. The tank is about 35L capacity, with OF water filter and air pump. The tank has a limited edition LOTR Lego set, gravel sand, and some stones (silicate based). Inside the tank I have a zebra crayfish, seven baby blue crayfishes, 3 yamato shrimps, and 20 tetras.

All went well for several weeks, with twice weekly 30% water changes. Then one night after work, I noticed that the water is cloudy, with slight white froth on the top of the water, and the fishes are behaving weirdly. I immediately initiated a 30% water change (the previous water change is just 1 day before), thinking everything will be fine. The following morning when I woke up, all the fishes died. The water has this weird chlorine / burnt cigarette butt smell, and I immediately did a 80% water change. When I came back home that evening, all the baby crayfishes died, and subsequently the zebra crayfish also succumbed to the condition.

Not knowing whats the condition, I cleaned the tank thoroughly, washed all parts that can be dismantled, soaked everything that can be dismantled into hot, near boiling water (including pump parts and stones). The water is totally changed and finally the whole tank is setup to its original configuration. However, several days after having nothing living in the tank, there is still some slight burnt / chlorine smell whose origins / exact smell cannot be traced. One week passed after the die out, I purchased some tetras, and 2 of my remaining crayfishes into the tank. The tetra seems fine on the first day, but are found to again behaving weirdly on the second day, now I promptly removed everything into spare tanks.

Some other background information:
Water used is from the tap, all water used are pre-treated with anti-chlorine and are negative to standard chlorine test kit.
pH of tank is measured with pH meter and averages around 7.8
Test kits for Nitrate, Nitrite, GH, KH, ammonia all turned up normal on every single tests done

The question is, whats wrong, and what can be done to turn around the situation?

Mysterious happening.

[Urban Aquaria]

Just to confirm that the tank was tested to be fully cycled with stable parameters before livestock were added in?

Possible cause could be the high bio-load in that tank pushed the capabilities of the filtration and beneficial bacteria in the filter media over their tipping point and ammonia spiked up suddenly (the bacteria cannot handle anymore of it), as ammonia exists in more toxic form due to the higher pH water conditions, most of the livestock are affected and killed by the combination of ammonia poisoning and sudden change in parameters from the large water change.

Cloudy water usually indicates a bacterial bloom, which tends to follow an ammonia spike. Froth forming on the water surface indicates very high levels of organic compounds. When this happens in a tank with livestock, its already a warning sign.

Its possible that ammonia can suddenly spike up, kill the livestock, then drop back down again (the bacteria simply took longer to process it, but eventually did), and all this can happen in the space of a few hours... so when you test the parameters after the incident, it seems okay.

After you washed and boiled all the tank items, it is clean but all the beneficial bacteria also got killed in the process, so your tank become like a new setup again and go back into cycling mode. Hence when you add new livestock in, they enter an uncycled tank which probably cause those fishes to react strangely.

As for the odd smell coming from the tank water, it could be residual organics that are still present in the equipment or the same issue starting to appear again. No idea if the plastics in the Lego set could have leached toxins into the water due to chemical reaction with anti-chlorine or the bacteria consuming the material (thats the big unknown). Smell is subjective though so its tough to figure out based on individual description.

Anyways, some general recommendations to help avoid or minimize such things happening in the future:

1. Increase tank size (More water volume = more stable parameters and longer time for toxic parameters to accumulate).
2. Reduce bio-load (Less livestock = less waste being produced, easier for filtration to handle).
3. Increase filtration and filter media volume (More filter media = more places for higher population of beneficial bacteria to live in and handle the bio-load).

Btw, what type and brand of test kits are you using?

[fireblade]

a picture of your tank might help
what is OF water filter ? Ocean Free band?

I will suggest you to remove the Lego set first.
what filter media are you using?

[Laika]

Hi, I'm sorry about the fish but a tank that size you should only really keep maybe 4 or 5 tetra and that's it. The ammonia and waste would of been the killer here I'm afraid

[happydanio123]

That is definitely a case of overstocking. Upgrade to a bigger tank and aquire a bigger filter for better results.