FAQ: Plants for fish

A diffuser breaks CO2 gas into very fine bubbles that rise slowly and dissolve in the water. It is best placed under the filter outflow or near the bottom so the bubbles stay in the water longer and spread around the tank.

Too much CO2 is dangerous for fish: they struggle for breath, hang at the surface and breathe rapidly. Increase the supply gradually, watch the fish, use a CO2 drop checker and switch the system off at night with a timer and solenoid valve.

It is a small glass vial with indicator fluid that changes colour with the CO2 level in the water: blue means too little, green about right and yellow too much. The colour changes with a lag of a few hours, so adjust the supply gradually.

Usually yes, because plants do not use CO2 in the dark and fish need more oxygen at night. CO2 is typically switched on an hour or two before the lights and off an hour before they go out, controlled by a solenoid valve on a timer.

It depends on the cylinder size, the tank volume and the bubble rate: a small cylinder on a modest tank can last several months, a larger one longer. Check the fittings for leaks, as that is the most common reason a cylinder empties fast.

A yeast generator is a simple way to try CO2 on a small tank, but its output varies and the mix must be replaced every few weeks. It cannot be switched off at night, so do not raise the output so far that fish run short of oxygen.

Yes, dissolved CO2 forms carbonic acid and lowers pH, typically by about one unit during the day. This is normal and harmless to most fish when the change happens gradually each day, though in hard water pH drops less.

Algae usually appear when CO2 is unevenly spread, the supply fluctuates or the light is too strong relative to CO2 and fertiliser. Check the flow, make sure CO2 starts before the lights and balance lighting with fertilising.

Yes, strong aeration and a rippling surface drive some CO2 out of the water. In tanks with a CO2 system the air pump is usually only run at night, while in tanks without CO2 gentle surface movement does not bother the plants.

Most aquariums do well with 6–8 hours of light a day, and in a new tank it is better to start shorter and extend it gradually. Lighting for too long does not help the plants but strongly encourages algae.

Plants do well with full-spectrum LED aquarium lights whose strength matches the plants’ needs. Undemanding plants are fine with low or medium light, while carpeting and red plants need strong light, usually together with CO2.

A standard lid light is usually enough for undemanding plants such as anubias, Java fern or cryptocorynes. For carpeting and light-hungry stem plants it is normally too weak, so a stronger plant-oriented LED light is needed.

Plants do well with a full spectrum including the red and blue parts they use most for photosynthesis, and a typical colour temperature is around 6000–7000 K. The colour balance mostly affects how the tank looks; for growth, light strength matters most.

It is very useful, because plants and fish benefit from a steady daily rhythm, and a light left on by mistake encourages algae. Many LED lights have a built-in timer with gentle sunrise and sunset; others work with a simple plug-in timer.

A siesta is a break of a few hours in the middle of the day when the lights are off, splitting the light period in two. Some aquarists use it to curb algae and let CO2 build up, but it is not necessary.

Direct sunlight heats an aquarium strongly and lights it unevenly, so it usually causes an algae bloom. It is better to place the tank away from windows and give plants controlled artificial lighting.

Plants lack light when stem plants grow leggy, the gaps between leaves widen, lower leaves die off and red plants turn green. In that case extend the light period or strengthen the lighting gradually, watching for an increase in algae.

Light that is too strong without enough CO2 and fertiliser does plants no good and usually causes algae, while shade-loving plants may fade or get covered in algae. Match light strength to the plants’ needs and the nutrient supply.

Fluorescent tubes fade over time, so they are often replaced every year to eighteen months. LED lights last much longer, but are worth replacing if the light has noticeably weakened or some of the LEDs have failed.

In tall tanks deeper than about 60 cm, light weakens before reaching the bottom, so carpeting and light-loving plants need a stronger light. Shallower tanks make plants easier to grow and cheaper to run.