Why are my fish dying?

[Byron]

Ok unfortunately my last goldfish didn't make it through the night. I'm really frustrated and sad and I wish I at least knew what the problem was so I could fix it.
I tested water and got
pH 6.4
Ammonia 0.1
Nitrite 0
Nitrate 10-20

I am leading towards something toxic in the tank. Everything EXCEPT the gravel was already in the tank. So maybe gravel is a good place to start.

IF it is the gravel, how would I best deal with this?

I was actually going to suggest a tear-down of the tank. I had to do this once, due to an unknown toxin when nothing else worked.

Drain the tank, discard the gravel (you can use this in your landscaping perhaps, if you have a house) and the decor. For substrate I would suggest play sand; I have this in all my 8 tanks now, and wish I had gone to it much earlier. I don't know where you live, but in NA you can buy Quikrete Play Sand at Home Depot and Lowe's. A 25 kg bag cost a few dollars, and would be enough for your 200 liter tank, though a second bag for a bit extra wouldn't hurt. For decor, I would get some chunks of wood, like Malaysian Driftwood.

Also replace all removable media from the filter, as this could pick up toxins.

Byron.

 

[Anankee]

I was actually going to suggest a tear-down of the tank. I had to do this once, due to an unknown toxin when nothing else worked.

Drain the tank, discard the gravel (you can use this in your landscaping perhaps, if you have a house) and the decor. For substrate I would suggest play sand; I have this in all my 8 tanks now, and wish I had gone to it much earlier. I don't know where you live, but in NA you can buy Quikrete Play Sand at Home Depot and Lowe's. A 25 kg bag cost a few dollars, and would be enough for your 200 liter tank, though a second bag for a bit extra wouldn't hurt. For decor, I would get some chunks of wood, like Malaysian Driftwood.

Also replace all removable media from the filter, as this could pick up toxins.

Byron.

Ok thanks Byron, I'll give that a go.

This means I have to start the cycle again right? From what I understand , I can use about 20% of the media from my other tank to help get it going.

Can I start this process without any sand / gravel or decor? I probably won't have a chance to get any for a couple of weeks due to work / public holidays.

Also, do I need to look out for any "nasties" when buying play sand? Do I just rinse it really well ad with any other decor before putting it in the tank?

Really annoying; the gravel you see in the photos above cost $120 in my LFS

I'm also a little hesitant to throw out my current driftwood and rocks because they belonged to my brother who recently passed . I will if I have to, but just wondering if there is anyway to save them? The gravel is just money, but these are sentimental


Thanks so much for the advice. As a newcomer to all of this it is really helpful! I am loving my fish! I had no idea that fish can have such great personalities. I can (and do) sit there and watch them for hours on end. They are absolutely beautiful and I really want to make a great (and safe) home for them in a decent size tank . Even though they are doing well where they are, they definitely need more room

 

[Byron]

Play Sand is frankly the safest substrate we can use. It is refined more than other sands so it is not rough, and it looks natural. It tends to need good rinsing at first to remove the fine dust/dirt, but even if you are not too thorough, this will not harm fish. Aquarium sand purchased in a fish store is also safe (hopefully...) but extremely expensive.

As for the wood and rock...without knowing just what is causing the fish deaths, you will run a risk using anything now in the tank. If the wood has absorbed some toxin, it will not rinse off. But at the same time, we've no way of knowing just what may or may not be present.

And this goes even more for filter media used to speed up the cycling. Don't. It is easy to cycle the tank as a new aquarium.

Byron.