The Sticker Gallons Lie
A "20-gallon" tank rarely holds 20 gallons of water. The glass box might displace that much, but you did not fill it to the brim, and the substrate, rocks, driftwood, and filter intake all occupy space where water — and medication — would otherwise be. The standard correction: true gallons = (L × W × H ÷ 231) × fill% × (1 − displacement%). A 24×12×16-inch tank at 90% fill with 10% displacement holds 16.2 gallons, not 20 — dosing for 20 overdoses by nearly 25%.
This is the single most common dosing error in the hobby, and it is completely silent: the fish look fine for an hour, then gasp at the surface. The calculator above forces the honest volume first, because every dose below is computed from it. When in doubt between two displacement estimates, use the higher displacement — underdosing slightly is safer than overdosing, and you can always extend treatment.
How the Common Medications Work
Ich-X (formalin + malachite green): the workhorse for ich (white spot) and many external parasites — 5 ml per 10 gallons, dosed daily. It only kills the free-swimming stage of the parasite, which is why the calendar runs 7+ days even after spots vanish. API General Cure (praziquantel + metronidazole): 1 packet per 10 gallons, dosed on day 1 and day 3, for internal parasites and gill flukes. Seachem KanaPlex (kanamycin): 1 scoop per 5 gallons every 2 days, up to 3 doses, for bacterial infections like fin rot and popeye. Maracyn (erythromycin): 1 packet per 10 gallons daily for 5 days, another bacterial option. Aquarium salt: 1 tablespoon per 3 gallons as a one-time tonic dose — dissolved first, removed gradually through water changes.
Every one of these has the same fine print: remove activated carbon (it absorbs the medication), turn off UV sterilizers, keep biological filtration running, and increase surface agitation — most treatments reduce dissolved oxygen, and sick fish need more of it, not less.
Reading the Treatment Calendar
Medication schedules exist because parasites and bacteria have life cycles. Ich cysts on the fish are armored against treatment; only the free-swimming theronts die — so you dose across multiple days to catch each hatching wave. Stopping when spots disappear (usually day 3–4) is the classic failure: the next wave hatches into an unmedicated tank and the outbreak "comes back." The calendar marks water-change days (25% before redosing) because many meds accumulate — each change resets the concentration so day 5 is as safe as day 1.
Log everything: dose amounts, water-change volumes, temperature, and fish behavior. If a second course becomes necessary, that log is the difference between a confident adjustment and a guess. The calendar above is a plan, not a prescription — if fish worsen sharply mid-course, stop, do a large water change, and reassess the diagnosis before continuing.
Overdose: Signs and Rescue
Overdose signs appear fast — often within an hour of dosing: gasping at the surface, clamped fins, erratic darting, loss of equilibrium, sudden deaths. Do not wait to confirm. The rescue protocol: immediate 50% water change with temperature-matched, dechlorinated water; add fresh activated carbon to pull remaining medication; maximize aeration; dim the lights to reduce stress. Then figure out what went wrong — nine times in ten, it was sticker gallons.
Some species are canaries: scaleless fish (loaches, catfish, eels), invertebrates (shrimp, snails), and labyrinth fish are sensitive to many meds at label doses. If your tank includes them, research species-specific dosing first — several products recommend half-doses for sensitive tanks, and copper-based treatments are outright lethal to invertebrates.
Salt: The Underrated First Response
Before reaching for the medicine cabinet, consider salt. A tonic dose of 1 tablespoon per 3 gallons eases osmotic stress, knocks back many external parasites, and supports slime-coat recovery — with a safety margin far wider than pharmaceuticals. Dissolve it completely in removed tank water before adding (undissolved crystals burn fish that investigate them), dose once, and let water changes dilute it out over the following weeks.
Limits: many live plants melt in salt, and scaleless fish tolerate it poorly — use half doses or skip it there. Salt is a support therapy and mild parasiticide, not a cure for advanced bacterial infections. Think of it as first aid while you confirm the diagnosis, not a replacement for proper treatment.
Quarantine: The Tank That Pays for Itself
The cheapest medication is the one you never need. A bare 10-gallon quarantine tank — sponge filter, heater, hide — costs less than one round of lost livestock and lets you observe new fish for 2–4 weeks before they meet your display tank. Most outbreaks trace to a new arrival; quarantine breaks that chain. Treat prophylactically in quarantine if you like (many keepers run a salt or General Cure course on all newcomers), because treating 10 true gallons in a bare tank is simpler, cheaper, and safer than treating a planted 75-gallon display.
When Not to Medicate
Medication is stress, and stress kills sick fish as surely as disease. Do not medicate for: a single odd behavior with no physical symptoms (observe 24 hours first), water-quality problems (test ammonia, nitrite, nitrate — most "disease" is actually poisoning, and no medication fixes that), or old age. The diagnostic order is always water test → observation → identification → treatment. A $15 liquid test kit prevents more deaths than every medication combined.
And never mix medications unless a trusted reference specifically endorses the combination. Stacked treatments multiply stress and can interact chemically. One diagnosis, one course, water changes between — the calendar above enforces exactly that discipline.
Dosing Small Tanks and Hospital Tanks
Small volumes punish sloppy math. In a 5-gallon betta tank, a 1 ml measuring error on a 2.5 ml dose is a 40% overdose — the same error in a 75-gallon tank is a rounding blip. For anything under 10 true gallons, dose with a 1 ml syringe (no needle, from any pharmacy), not the cap or a teaspoon, and compute from the true volume the calculator gives you, never the sticker size. Better yet, treat sick fish in a bare hospital tank: a simple 10-gallon with a sponge filter and heater. Dosing is simpler, water changes are trivial, the main tank's biological filter and plants stay unmedicated, and you can observe the patient without eleven tankmates photobombing the diagnosis.
Hospital tanks change the economics of treatment too: medicating 8 true gallons costs a fraction of dosing a 60-gallon display, and daily 50% water changes — the gold standard during treatment — take ten minutes instead of an hour. Keep a cycled sponge filter running in your main tank or sump at all times; when illness strikes, you have an instant, safe hospital setup instead of a panic purchase. The fish you save may be your favorite — they usually are.
After Treatment: Carbon, Water Changes, and Watching
Finishing the last dose is not the finish line. Run fresh activated carbon for 48–72 hours to pull residual medication, then do a 30–40% water change and test your parameters — treatments often dent the biological filter, and a mini-cycle in the week after medication kills more fish than the original disease. Watch appetite first: a fish that eats aggressively within 48 hours of treatment's end is recovering; one still hiding and clamping needs reassessment, not another round of the same drug.
Wait at least a week of clean water and normal behavior before declaring victory, and resist restocking immediately — new arrivals during the recovery window meet stressed tankmates and leftover pathogens. Log the whole episode (disease, product, doses, dates, outcome) in a notebook or phone note; six months later, when something looks familiar, that log is worth more than any forum thread.