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Start with what your tank is actually short of
Plants need macronutrients - nitrogen, phosphorus, potassium - and a long list of micronutrients and trace elements. The fertilizer aisle is built around tanks that are short of the macros, because that is what a heavily planted, lightly stocked, CO2-injected display tank runs out of first.
A stocked nano tank is not that tank. You have a fish in a small volume, fed daily. Nitrogen arrives continuously as ammonia, gets processed to nitrate, and accumulates until you change water - the whole reason nitrate control is a maintenance job is that a stocked tank makes more of it than slow plants can use. Phosphorus arrives with the food.
So in the tank this site is written for, dosing a macro-heavy all-in-one is adding more of what you already have too much of. What the plants are genuinely short of is iron and the trace elements - and in the case of root feeders, the nutrients they take from the substrate rather than the water.
What we ranked these on
We have not run growth trials. There is no test bench here, so every criterion below is either published by the manufacturer or is arithmetic you can redo.
Published dosing precise enough to convert. This is the criterion that decides the page, and it is why both picks are Seachem. A label that says “a capful per tank” cannot be scaled to 4.3 gallons of water; one that publishes milliliters per 60 gallons can.
Honest about what it omits. Seachem states plainly that Flourish does not contain significant nitrogen or phosphorus. That disclosure is worth more than a longer ingredient list, because it tells you which half of the problem the bottle solves.
Suits a low-tech, stocked tank. Not a show tank. The plants we recommend are published at 10-20 PAR with no CO2 requirement, and they grow slowly by design.
Nano dose usability. Whether the correct dose is something a human can measure. This is where liquid fertilizers struggle and tablets win.
Value per gallon treated. At nano volumes almost anything lasts years, which makes the smallest bottle the right purchase nearly every time.
The nano dose, worked
Seachem publishes 1 capful - 5 mL - of Flourish for each 250 L / 60 US gallons, once or twice a week. Convert that to the tanks this site covers and you get the same faintly absurd result as the dechlorinator.
| Tank | Actual water | Flourish per dose |
|---|
| Rated 5 gal | 4.3 gal | 0.36 mL |
| Rated 10 gal | 8.5 gal | 0.71 mL |
| Rated 20 gal | 17 gal | 1.42 mL |
A third of a milliliter. The cap holds 5 mL, so in a rated 5-gallon you are being asked for roughly one fourteenth of a capful. Use a 1 mL syringe - the same one the conditioner needs - and note the one difference from Prime: Seachem publishes no overdose tolerance for Flourish, so the “a few drops is close enough” shortcut that works for a dechlorinator is not sanctioned here. Measure it.
The tabs dose is published as a spacing, which is better
Seachem gives Flourish Tabs as one tab per 10-15 cm / 4-6 inch radius, and states that a standard 10-gallon aquarium requires six. That is a rule you can apply to your own footprint before you buy: a standard 10-gallon base is 20 by 10 inches, so six tabs covers 200 square inches - roughly one tab per 33 square inches. A Fluval Spec V base of 19.5 by 6.5 inches is about 127 square inches, which comes out at four tabs. Replace every three to four months, per Seachem.
Water column versus roots: the split that decides your purchase
Plants take nutrients in two ways, and the two products here are not competitors - they address different halves.
Rhizome plants and moss feed from the water column. Anubias, java fern and java moss attach to hardscape and have no meaningful root system in the substrate. A liquid supplement is the only way to reach them, and root tabs under them do precisely nothing.
Crypts and grasses feed from their roots. Aquarium Co-Op names root tabs or a nutrient-rich substrate for both Cryptocoryne wendtii and dwarf sagittaria. A liquid dose in the water column reaches those plants far less efficiently.
So the honest answer for most readers of this site is: look at what you actually planted. A tank of anubias and java fern tied to wood wants the liquid and nothing else. A tank with crypts rooted in gravel wants the tabs. A mixed tank wants both, and that is a perfectly reasonable place to end up.
If your substrate is already a soil, wait
Fluval Stratum and Bio Stratum are nutrient-bearing soils, and a new tank set up on one has a nutrient reserve in the substrate already. Adding root tabs on day one is spending money to solve a problem you do not have yet - and in a nano tank, surplus nutrients in the water are an algae input rather than a plant input.
The published guidance from Aquarium Co-Op on crypts is the useful framing: add root tabs when leaves yellow and die back after a couple of months, which is the plant telling you the substrate reserve has run down. Which soil you chose determines how long that takes.
The algae trade nobody puts on the bottle
Every nutrient you add is available to algae as well as to plants. In a tank with fast-growing, well-lit plants the plants win the competition. In a low-tech nano full of slow-growing rhizome species under a modest light, they may not.
That is the real reason this page recommends restraint rather than a dosing schedule. Start at the published dose once a week rather than twice, and watch what happens for a month. If the plants are pale and new growth is small, go to twice. If the glass starts greening faster than it did, go back - and shorten the photoperiod before you change anything else, because light is usually the lever, not fertilizer.
What we did not rank, and why
All-in-one fertilizers with macros. They are the right product for a high-tech planted tank and the wrong one for a stocked nano, for the reason this page opens with. If your tank has no fish in it, this recommendation inverts.
Aquarium Co-Op Easy Green. It is an excellent, widely recommended all-in-one with published dosing, and we could not resolve it to a verified Amazon listing - it is sold direct. Rather than point you at a third-party reseller we cannot verify, we have left it off. If you already buy from Co-Op, it is a reasonable alternative to the liquid pick here, with the same macro caveat reversed: it does carry nitrogen.
Liquid carbon products. Not fertilizers. We set out what we will and will not say about them on the CO2 page.
DIY dry salts. Cheaper per dose by a wide margin and genuinely the right answer for someone dosing several tanks. At a single nano tank the weighing equipment costs more than years of bottled supplement.
What we cannot tell you
Whether your plants are actually deficient. Nutrient deficiency in aquatic plants is diagnosed from leaf symptoms, and the published symptom guides disagree with each other often enough that we are not going to add another. What we can tell you is what each product contains, what it omits, what the manufacturer says to dose, and what that works out to in 4.3 gallons of water.
