Water, temperature and the nitrogen cycle

The numbers to aim for

Flowerhorns come from Central American cichlid stock, fish that live in warm, moderately hard, slightly alkaline water. They adapt to a fairly wide range, so there is no need to chase a perfect reading. What matters is that the numbers stay stable and the waste stays low.

You will find slightly different figures elsewhere. Some references list temperatures down to about 77°F (25°C) or a pH as low as 6.0. A flowerhorn will survive those conditions, but the ranges above are where we see the best appetite, color and activity.

Temperature: warm and steady

Fish are cold-blooded, so water temperature sets the pace of everything: appetite, digestion, growth and immune response. In the low to mid 80s a flowerhorn eats well, grows steadily and shows its best color. Let the tank drift into the low 70s and most fish turn sluggish, dark and uninterested in food.

Warmer is not always better, though. Warm water holds less oxygen, and a larger share of any ammonia present is in its most toxic form at higher temperatures. Pushing a tank toward 90°F (32°C) to speed up growth trades a little extra metabolism for a lot less safety margin. If you run the top of the range, add surface agitation or an air stone.

Stability matters as much as the number. Try to keep daily swings within a degree or two. The usual causes of swings are a failed or undersized heater, a tank near a window or vent, and water changes with new water that is noticeably colder or warmer than the tank. Heater choice and placement are covered in the tank setup guide.

pH and hardness

Plenty of municipal tap water in the US, including much of Southern California, is on the hard, alkaline side. That suits flowerhorns, and most keepers simply use dechlorinated tap water. Your water utility’s annual water quality report lists hardness if you want to check, or test it yourself.

A few principles keep chemistry from becoming a problem:

• Stable beats perfect. A steady pH of 7.2 is better than one bounced between 7.0 and 8.0 by chemical adjusters. Avoid pH-up and pH-down products unless you understand what is causing the reading.
• Watch for a slow drop. Waste breakdown produces acids. In a tank that has gone too long between water changes, pH creeps down as carbonate hardness (KH) is used up. Regular water changes prevent it.
• Soft or acidic water needs a buffer. If your source water is soft, a bag of crushed coral or aragonite in the filter raises hardness and steadies pH gradually.
• Reverse osmosis water needs minerals. Pure RO water is too soft for a flowerhorn on its own. Remineralize it or blend it with tap water.

The nitrogen cycle, in plain terms

A flowerhorn’s gills and waste release ammonia, and leftover food adds more as it rots. Ammonia is toxic. In a healthy tank, one group of bacteria living in the filter converts it to nitrite, which is also toxic, and a second group converts nitrite to nitrate, which is far less harmful and is removed with water changes. Growing those bacteria before the fish arrives is called cycling.

This matters more for flowerhorns than for most fish. University of Florida extension scientists note that un-ionized ammonia, the dangerous form, is roughly 100 times more toxic than the ionized form, and its share rises as pH and temperature rise. Warm, alkaline water is exactly what a flowerhorn lives in, so a small ammonia reading does more damage here than it would in a cool, acidic tank.

1. Set up and dechlorinate

   Fill the tank, add a conditioner that handles chlorine and chloramine, and run the filters and heater at normal temperature.

2. Add an ammonia source

   For a fishless cycle, dose pure household ammonia with no surfactants or scent, or add a small amount of fish food daily. A bottled nitrifying bacteria product can shorten the process.

3. Test every two to three days

   Ammonia rises first, then nitrite, then nitrate appears as the second group of bacteria takes hold.

4. Confirm it is done

   The cycle is complete when the tank takes an ammonia dose down to zero ammonia and zero nitrite within about a day, with nitrate showing. Commonly this takes three to eight weeks, sometimes less with bottled bacteria or media from an established filter.

5. Change water, then add the fish

   Do a large water change to bring nitrate down before the flowerhorn goes in, and feed lightly for the first week.

A new flowerhorn in an uncycled tank is one of the most common problems we hear about. Symptoms of ammonia or nitrite stress, and what to do about them, are in the diseases guide.

Water changes that work

Filtration turns ammonia into nitrate, but only water changes take nitrate and dissolved waste out. For an adult flowerhorn, 25 to 30 percent every week is the routine that keeps most tanks in good shape. Tanks on the small side, or fish that are fed heavily for growth, often need more, either larger changes or two a week.

• Siphon the bottom. Pull uneaten food and waste off the glass or sand, not just water from the middle.
• Match the temperature. Bring new water within a degree or two of the tank. Sudden temperature drops stress fish and are commonly blamed for ich outbreaks.
• Treat all new water. Dose conditioner for the full tank volume when adding water directly from a hose, since many utilities use chloramine.
• Catch up gradually. If a tank has been neglected for weeks, do several moderate changes over a few days rather than one huge one. A sudden shift in pH and hardness can shock a fish that has adapted to poor water.
• Leave the filter alone that day. Clean filters on a different day from big water changes so the bacteria are not disturbed all at once.

A testing routine

A liquid test kit costs less than one good flowerhorn and is more accurate than most strips. Keep a simple log of dates and results.

If ammonia or nitrite reads above zero in an established tank, do a 30 to 50 percent water change, feed lightly or skip a day, and look for the cause: a dirty or recently over-cleaned filter, a heavy feeding, or something decaying in the tank. For diet habits that keep the water clean, see the feeding guide, and for the full overview go back to the flowerhorn care guide. San Diego keepers are welcome to bring a water sample to Pet Zone.