Determining the bacterial count - DIYS

Short how-to for making a culture medium that may give you insight over the bacterial count in your aquarium water.

by Bernhard Ehmer, translated by Ulli Bauer

What we need:

• 5 potatoes
• 100 ml water
• 2g powdered agar (no Agartine)
• 1 kitchen towel
• Petri dishes with lid (glass)
• Scotch tape
• pressure cooker (a baking oven will also do)

Step 1:

Cook the potatoes (unpeeled) for around 30 minutes, until done 

Step 2:

Mash cooked potatoes.

Step 3:

Put the soft mash into a kitchen towel, make a sack-like bundle and press the mashed potatoes through the towel into a small cooking pot (as you would do when making potato dumplings).

Step 4:

Add the agar to the pot with the liquid you got from pressing the potatoes, and boil for 30 seconds (don't forget to stir).

Step 5:

Fill the hot agar-potato-mixture into the Petri dishes. A layer of 3 mm height is sufficient. (If your mixture is foamy, pour through a fine tea sieve, the foam will be held back in it).

Step 6:

Envelop Petris dished carefully in aluminium wrap (individually!) and sterilize for a good 40 minutes in your pressure cooker.

Step 7:

Let the pressure cooker cool down, this is best done outside. After it's cool, take out the Petri dishes and seal them with Scotch tape.

For measuring the bacterial count: fill 5 ml of tank water into your Petri dish and pour it out again at once. Close with the lid immediately, seal with Scotch tape and put in a warm place (25-35 Grad Celsius). After 24h you should be able to spot the first bacteria colonies.

Do not breath on the culture medium, this would falsify your results!

How to interpret the contamination:

100 000/ml could rightfully be called a catastrophe in the making. 1000 might be a very positive result (might be, as we do not know exactly which bacteria are present).

Important:

1. Never open contaminated Petri dishes!
2. Remove the Scotch tape from contaminated glass Petri dishes and put them in the oven at 120 to 150 °C for at least 30 minutes.
3. Rinse out the dead colonies and the culture medium with very hot water.
4. Clean the Petri dish with a disinfectant or high-proof alcohol.

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