Article: Balancing the needs of fish and plants

Plants are a joy to keep, and the properly maintained planted tank is one of the most impressive sights in fishkeeping. Making the change from only considering the fish to thinking about the needs of the fish and plants can be a balancing act there at the beginning though!


pH, KH, and GH

The first and possibly most important factor to consider is tank chemistry. Many first-time plant keepers are surprised that “ideal” tank parameters for fish aren’t necessarily “ideal” water parameters for plants. Discus, for example, prefer a pH, KH, and GH so low that they’re difficult to maintain even without the interference of aquatic flora. By contrast, fish like mollys like a very high GH, KH, and pH that can be quite stressful to plants. There are plenty of plants that are suited to either of these environments, but choosing the right ones can take a bit of time and research.



Once a set of water parameters are found that will suit every tank inhabitant, the question of what products to use for those parameters comes into play. The product that is best for the fish might not be the best for the plants, and vice versa. The strongest example in the Seachem® line is Replenish™ and Equilibrium™ for GH.
  • Replenish™ has all the minerals the fish need for proper osmoregulation, is easy to dose and use, and contains sodium chloride that will help prevent nitrite poisoning (much more dangerous than ammonia at low pHs). However, sodium chloride can be stressful to plants if enough is built up in the water, so we don’t recommend it for planted tanks.
  • Equilibrium™ is much better suited for planted tanks, as the minerals are in the preferred form for plants, it contains no sodium chloride, and has extra potassium and iron for uptake by the plants. Fish don’t need the extra potassium and iron though, so while it won’t be stressful to them, it’s not necessary for their health either. This is a powder supplement too, and it can take some time to dissolve it fully in water, unlike liquid supplements.
  • Mineralize™ from the aquavitro® line is liquid, so is much easier to dose and dissolve than Equilibrium™, and is what we usually recommend for people who are having trouble making the shift from powder supplements to liquid supplements.

KH & pH

KH and pH are an entirely different kettle of fish (and plants). If a planted system has a CO2 injection system, then the fish keeper will need to account for this by maintaining a higher alkalinity (or KH, depending on your test kit) to prevent the pH drop associated with CO2. This sort of issue can be avoided by instead using a liquid carbon supplement like Flourish Excel™, but many fish keepers still like to use CO2 for heavily planted tanks. On top of the difference in “ideal alkalinity”, there are different buffering systems recommended for planted systems and fish-only systems.
  • Neutral Regulator® is an excellent buffer for the fish-only tank. It’s strong, stable, doubles as a water conditioner and ammonia detoxifier, and is simple to use. It’s also phosphate based. Phosphate is a nutrient for both plants and algae, so combining high levels of phosphate with the high lights of a planted tank can exacerbate algae issues. In a fish-only system we can control algae by turning off the lights or bringing nitrate to 0, but in a planted tank that’s not an option.
  • Alkaline Buffer™ and Acid Buffer™ are carbonate-based buffers for the planted system. Carbonate based buffers won’t interfere with the all-important NPK ratio of plant nutrients, and so are preferred for planted tanks. They work great for fish-only systems too, but aren’t quite as stable and easy to use as the regulators. For example, they need to be used together to maintain a neutral pH, while it’s possible to fine-tune the phosphate-based “regulators” to the point that we can target a neutral pH with one product alone.


Once pH, GH, and KH are set, the question of nutrients comes to the fore. Fish tend to be happiest with minimal nitrogenous waste, but plants need a consistent nitrogen source in order to grow well (nitrogen is the “N” part of the “NPK” ratio that was mentioned earlier). That means that fish keepers who are used to doing a water change the very moment a whiff of nitrate threatens their delicate discus will need to rearrange their expectations a bit to accommodate the needs of the plants. Similarly, plants need consistent levels of phosphate and potassium, although these are not a threat to the fish in standard aquarium concentrations. There shouldn’t ever be a need to dose nitrate, phosphate, or potassium supplements in a fish-only tank, but for planted tanks we like Flourish Nitrogen™ or aquavitro synthesis™ for nitrogen, Flourish Phosphorous™ or aquavitro activate™ for phosphorous, or Flourish Potassium ™for potassium (the potassium in the aquavitro® line is spread across several products).
Of course, aside from the macroelements plants also need micro and trace elements. Fish aren’t really bothered by these unless they rise very suddenly (always follow the dosing instructions on the bottle), so when used as recommended, most plant supplements work perfectly well with fish. We like Flourish® for micro and trace elements in tanks, or you can dose separately with Flourish Iron™ and Flourish Trace™.



Substrate is a topic that hardly even enters the conversation when it comes to fish-only systems but suddenly becomes vitally important when we talk about plants. With a few notable exceptions (catfish and other bottom feeders) fish don’t particularly care about the appearance or chemical composition of their substrate as long as it is tank safe and is not pushing their major water parameters in an undesirable direction. Plants on the other hand, especially heavy root feeders like swords, care very much about the chemical composition of the substrate. Here are some suggestions for substrates that suit the plants and the fish:
  • Flourite® is a great all-around substrate. It won’t change the GH or KH, and it is safe to use in fish-only tanks. It comes in a variety of colors and textures, and will provide an excellent base of nutrients for plants as well!
  • Flourite Black Sand™ is a fantastic choice for tanks that include bottom feeders. It has all the nutrients and benefits of the Flourite® substrates, but the finer grain size lets fish dig through it easily in search of food.
  • Onyx Sand™ is well suited for harder-water tanks, like guppy or molly tanks. It also provides nutrients for the plants, or is great for a fish-only system.
  • Aquasolum® is a humate-based substrate from the aquavitro® line. It will soften and lower the pH and KH of aquarium water, so it is great for shrimp, dwarf cichlids, and other soft-water fish.


We’re covering lights last because there’s not really a conflict of interest here. Plants need a good spectrum and intensity of light in order to grow well, but fish hardly seem to notice the light at all as long as it is turning on and off at a regular schedule. Most companies will report the spectrum of their lights in Kelvin, and you’ll want around 5500K - 7500K. Of course, spectrum and intensity of light is much more complicated than that, but looking at the Kelvin rating will give you a pretty good idea of whether the light will suit your plants or not.
Both fish and plants need a set schedule of light for best health, and adjusting the intensity and duration of lights is often the key to algae control. There’s a point at which the plants stop benefiting from extra light and instead it just fuels more algae growth. We normally recommend 8-10 hours of light for plants, but this chunk of time can happen at any time of the day. For example, if you work during the day, it’s perfectly fine to have your aquarium’s “Day” cycle start just as you get home!


Be Proud

And with that, you’ve done it! Congratulations, you have joined a prestigious and elite group of Plant Keepers. Go forth, and photograph your tank! Post it on a forum! You’ve earned it.
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