Free Tools
No AI guesswork — structured diagnostics built from real symptom and cause data across 230+ species.
Diagnose Your Fish
Answer a few questions about symptoms and behavior and get 2-3 ranked likely causes, not a guess.
Compatibility Checker
Check whether species you're considering will actually get along in one tank.
Stocking Calculator
Bioload-based stocking guidance for your tank size — not inch-per-gallon myths.
Water Parameter Guide
Ideal ranges per species, plus a plain-language read on your own test results.
Disease Library
View all →From the Blog
View all →Types of Aquarium Algae: Identification and Fixes for Each
How to identify green dust, brown diatoms, black beard algae, hair algae, and cyanobacteria, and the specific cause-driven fix for each rather than a one-size-fits-all treatment.
Choosing Your First Fish Tank: Size, Shape, and Placement
Why a bigger first tank is usually easier, not harder, plus how tank shape affects stocking and where in the house a tank actually belongs.
How to Stock a Community Tank Without Overcrowding or Fighting
A practical framework for planning a peaceful community aquarium: understanding bioload versus the one-inch-per-gallon myth, matching temperament and water-level niches, and stocking order.
Why AquariumSOS
Most fish-problem searches end at a forum thread from 2014 or a generic "top 10 causes of fish death" listicle that never quite matches what's actually happening in your tank. AquariumSOS starts from a different premise: symptoms alone rarely point to a single cause, so instead of guessing, the diagnostic tool works through structured symptom-and-cause matching across a real dataset covering 230 species, returning ranked likely causes rather than one confident-sounding answer that might be wrong.
The same principle carries through the rest of the site. Disease guides walk through cause-by-cause treatment rather than one generic "add medication and hope" instruction, because ich caused by a temperature swing and ich caused by an unquarantined new fish call for different follow-up even though the treatment itself overlaps. Species pages go deep rather than wide — real adult size, real temperament, real water parameter ranges, not a recycled paragraph with the species name swapped in.
Beyond diagnosis, three free planning tools exist because most fish problems are more easily prevented than fixed after the fact: a compatibility checker that weighs more than "will they fight," a stocking calculator built around bioload instead of the flawed inch-per-gallon rule, and a water parameter guide that explains what your test kit numbers actually mean for the species you're keeping or considering.
None of it sits behind an account. Everything is built to be used once, get you an answer, and let you get back to your tank.
How It Works
Diagnose what's going wrong
Select the symptoms you're seeing — not just the most obvious one — and the diagnostic tool returns the 2-3 most likely causes, ranked, drawn from symptom patterns across 230 species and the disease library, rather than a single guess.
Check compatibility and stocking before you buy
Before adding a new fish, run it through the compatibility checker against your existing stock and the stocking calculator against your tank size and filtration, so problems get caught on paper instead of in the tank.
Follow species-specific care and disease guidance
Once you know what you're dealing with, species pages and disease guides give the specific detail — real parameter ranges, real treatment steps, real prevention advice — rather than generic care sheet filler.
230 species and 35 diseases documented — free, no account required.
Frequently Asked Questions
Is AquariumSOS actually free to use?
Yes. The diagnostic tool, compatibility checker, stocking calculator, water parameter guide, and every species and disease page are free with no paywall or usage limit.
Do I need to create an account?
No. Nothing on the site requires signing up or logging in — every tool and every page works the same for a first-time visitor as a returning one.
How accurate is the symptom diagnosis?
It's a ranked shortlist of likely causes based on symptom pattern matching, not a certain diagnosis — remote tools can't examine your fish or test your water directly. It's built to narrow down where to look first, particularly toward cheap, fast checks like water testing, not to replace hands-on veterinary assessment for serious cases.
Does this cover saltwater and reef fish, or just freshwater?
The current species database is predominantly freshwater, reflecting where most of the underlying research and data is strongest. Saltwater coverage is more limited for now, so treat saltwater-specific results with extra caution and cross-check against dedicated marine references.
What if my species isn't in the database?
The diagnostic tool and compatibility checker still work using general symptom and compatibility patterns even without a specific species match, though the results will be less precise than for one of the 230 species with a dedicated profile. If you keep running into gaps for a specific species, that's useful feedback for what to add next.