Aquarium fish are the most numerous pet in the world: it is estimated there are more than 700 million ornamental fish in homes worldwide. In Spain, fishkeeping is a growing hobby with a very active community. Its apparent simplicity — "they're just fish" — hides a more complex reality: well-practiced aquaristics combines biology, water chemistry, ecology and aesthetics. This article is aimed at those who want to start from scratch without making the most common mistakes.
The Nitrogen Cycle: The Most Important Concept
The number one mistake of beginner aquarists is introducing fish into a newly set up tank without having matured the biological filter. The result is "New Tank Syndrome": toxic ammonia levels that kill fish within days.
The nitrogen cycle works like this:
- Fish excrete ammonia (NH3), which is highly toxic.
- Bacteria of the genus Nitrosomonas colonize the filter and convert ammonia into nitrite (NO2), also toxic.
- Bacteria of the genus Nitrobacter convert nitrite into nitrate (NO3), much less toxic and removable through partial water changes.
This process (filter maturation) takes 4-8 weeks. It can be accelerated with commercial bacteria (Dr. Tim's, Seachem Stability), using water or filter media from an already matured tank, or starting the cycle without fish (fishless cycling) by adding pure ammonia. During maturation, parameters must be monitored with ammonia, nitrite and nitrate tests.
Choosing the Right Tank and Fish
Contrary to intuition, small tanks are harder to maintain than large ones: water parameters fluctuate more rapidly in small volumes. For beginners, a tank of at least 60-80 liters is recommended.
Fish recommended for beginners:
- Neon tetra (Paracheirodon innesi): small, peaceful, colorful. Live in groups of at least 6-8 individuals. Soft, slightly acidic water.
- Guppy (Poecilia reticulata): very hardy and tolerant of parameter variations. Very colorful males. Live-bearing.
- Platy (Xiphophorus maculatus): similar to guppy, peaceful and easy to care for. Great variety of colors.
- Corydoras: bottom fish that clean substrate debris. Essential in community tanks. Always in groups of at least 6.
- Danio (Danio rerio): very active, hardy and tolerant. Perfect for maturing tanks.
Fish to avoid as a beginner: goldfish (require cold water and lots of space), aggressive cichlids, large fish or those with very specific needs.
Water Parameters: What to Measure and Why
Tap water contains chlorine and chloramines harmful to fish. It must always be treated with a dechlorinator (Seachem Prime, API Stress Coat) before adding to the tank.
The fundamental parameters to monitor are:
- pH: most tropical freshwater fish prefer 6.5-7.5.
- Temperature: 24-27°C for most tropical fish. Requires a heater with thermostat.
- Ammonia and nitrite: always 0 ppm in a mature tank.
- Nitrate: <20-40 ppm. Controlled with weekly partial water changes of 20-30%.
- GH (general hardness) and KH (carbonate hardness): important for species with specific requirements.
Weekly Maintenance: Basic Routine
A well-established tank requires little weekly time if a routine is followed:
- Partial water change (20-30%): vacuum the substrate with a siphon to remove debris.
- Clean the glass with a magnetic scraper if there are algae.
- Check water parameters monthly with liquid test kits (more reliable than strips).
- Never clean filter media with tap water: use water from the tank itself to avoid killing beneficial bacteria.
Overfeeding is the most common cause of problems in beginner tanks. Fish should eat only what they can consume in 2-3 minutes, once or twice a day.
