The 5 fatal mistakes to avoid in aquarium keeping

Mistake no. 1: treating directly in the main tank

This is the classic blunder. You spot a sick fish, so you pour the product straight into the main aquarium.

• The consequence: you kill the "good bacteria" in your filter (see filtration), you stain the silicone and the decor permanently blue, and you needlessly stress the healthy fish.
• Best practice: always use a hospital tank (a simple clean bucket or a small bare 20 L tank with an air stone and a heater) to treat only the sick fish. It is cheaper, safer and more effective.

Mistake no. 2: forgetting to remove the activated carbon

You treat your fish, but after 3 days nothing has changed. Worse still, the water turns clear again within a few hours.

• The cause: your filter contains an activated carbon cartridge (often supplied by default in beginner kits).
• The consequence: the carbon adsorbs the medication instantly. You are not treating the fish, you are treating the carbon!
• Best practice: always check your filter and remove any chemical media (carbon, anti-nitrate resins) before a treatment.

Mistake no. 3: the "Molotov cocktail" mixture

"To be sure of killing everything, I'll add some blue, a little malachite green and some salt."

• The consequence: toxic synergy. Some products, once mixed, become lethal to fish (respiratory distress, burning of the gills). Methylene blue is a reducing agent and must not be mixed at random.
• Best practice: one treatment at a time. If you need to switch to another molecule, filter over carbon for 24 hours and carry out a large water change before introducing the new product.

Mistake no. 4: dosing "by guesswork"

"The water isn't blue enough, I'll add a little more."

• The consequence: although blue is of low toxicity, a massive overdose causes irritation of the mucous membranes and the gills. Conversely, underdosing breeds resistant strains of parasites.
• Best practice: work out the actual volume of your tank (Volume = Length × Width × Height - Decor). Use a syringe or a dropper to dose precisely according to the concentration of your stock solution (see dosage).

Mistake no. 5: underestimating the staining power

You open the bottle and a drop falls on the white lacquered cabinet in the living room or on the carpet.

• The consequence: an indelible stain that will remind you of this mistake for years. Methylene blue stains wood, plastic, clothing and tile grout instantly.
• Best practice: always work over a sink or a tray. Keep a damp cloth and some bleach within reach (for non-textile surfaces). Wear clothes that you do not mind ruining. (See stain management).

In summary: the safety checklist

Before treating:

1. I have isolated the sick fish (if possible).
2. I have removed the activated carbon from the filter.
3. I have calculated the exact volume of water.
4. I have protected my hands and my furniture.
5. I am not mixing it with another product.