How can you cheaply and simply check the oxygen content of water? .?

Question

e.g ok for fish to live in .

Answer 1

smell it, if it smells bad, change it. use bottled water, not tap.

Answer 2

Most of the oxygen in aquariums is found at the surface of the water. You can use a test kit to ensure the oxygen levels are between 5 and 7 ppm. Go to your local pet store and get one. If the tank has poor water circulation, an air pump and aerator can be used to increase oxygenation of the water.

Answer 3

If you have a well set-up aquarium (which has gravel, a water pump (with filtration), and turning water over, plants, heater etc),  and you have left the auarium running for a week or so (to allow the nitrogen cycle to start), then I suggest you do the following.

Purchase a few hardy fish Zebra Danio's or Tiger Barbs (or goldfish if it is for a coldwater aquarium and see how they do.  If you watch them for signs of distress and also if they are 'gulping' excessively at the surface.  If they are acting normal and healthy then you should not have a problem.  Low O2 levels in a aquarium are not usually a problem.

The most reliable method which is not cheap is using a Dissolved oxygen meter, but these are expensive and can be complicated to operate properly.  If you have real concerns you could take a sample of your water to a aquarium petshop (or perhaps a local friendly college) to test the water for you.

If I were you I would be more concerned about checking nitrate, nitrite, ammonia and pH levels regularly as these are the ones that can cause issue in a fish tank.  

Overstocking with fish can cause low O2 levels and also as water gets warmer it loses O2, so bear this in mind.  Excessive dirt in the tank can also cause low O2, but high nitrates would cause health issues before this.