Goldfish and the Nitrogen cycle

My son asked me earlier this year if he could have a goldfish. Sometime later he changed his mind and asked for a dog so we went out to buy a goldfish. Knowing nothing about goldfish we went to go buy one. The subsequent learning experience has had great relevance to my classroom. how-to-look-after-a-goldfish

Things I learnt

1. It is important to set up the aquarium and deposit food etc well in advance of the fish, anything from 7 days to two weeks (had no idea). This allows “good bacteria” to grow. I later found out that these “good bacteria” are nitrosomonas and nitrobacter and that the water is a veritable Nitrogen cycle. Goldfish excrete and egest a lot.

2. We had to buy a water filter and a 10 gallon tank. Remembering goldfish in a bag that I would get at the fair, this all seemed quite surprising.

3. Once the water had gone through a fishless cycle (This link refers to what we should have done not what we did) the water was taken to the pet store to be tested and then we were “allowed” to buy our fish. We supplemented the water with that from another aquarium to stock up on the bacteria etc.

Result:

Within a week the fish were struggling and one died. The ammonia level had gone through the roof (so to speak). I emptied half the tank (not all, as one does not want to remove all the bacteria 25 – 50% recommended) and kept testing for ammonia (very bad), nitrite (bad) and nitrate (okay) levels and the levels of each eventually reduced with repeated emptying. Sometime later we had started having problems with algal blooms brought on my either too much light or too much food in the tank.

I hope its clear that with dissolved ammonia, oxygen, nitrate and even CO2 sensors in a in-class aquarium, its possible to bring an ecosystem alive in your classroom and study the events realtime. Even studying / setting up the fishless cycle can be useful. Bringing in plants and having your students go online to explore what is required to set up a tank, looking at the different species etc. would be an excellent learning experience as it has been for me.

Next week I will write of my experience giving a DP bio category 1/2 workshop in New Orleans this weekend.

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