eSHa gdex vs ndx: Choose by Parasite Group
By Richard James · Updated 27 October 2026
Manufacturer information reviewed: 27 October 2026.
“Internal” versus “external” is an unreliable shortcut for choosing between these products. gdex includes tapeworms among its targets, so its use is not confined to the fish’s surface. A thin fish or a fish rubbing against decor needs investigation rather than an automatic product choice.
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The distinction that matters
| Product | Manufacturer’s target group | Plan to check |
|---|---|---|
| gdex | Specified flatworms, skin/gill flukes and tapeworms | Standard five-day plan; separate special or repeat instructions |
| ndx | Parasitic nematodes, such as Camallanus and Capillaria | Initial dose, subsequent cleanup and any indicated repeat |
Record observations without turning them into a diagnosis
Write down the species affected, appetite, body condition, breathing changes and visible material, with dated photographs where possible. Include water-test results and recent additions or treatments. Ask a fish-health professional what evidence would distinguish the suspected causes; apparent symptoms alone do not establish a parasite group.
The former explanation that gdex dissolves a parasite’s “armour” has been removed. No mechanism or comparison trial was presented to support it. We have also removed the universal “gasping means flukes” and “stringy faeces means worms” shortcuts.
Keep the schedules separate
Use the corrected gdex schedule for the standard arithmetic. For ndx, eSHa describes cleanup after 24 hours and a later repeat for egg-laying nematodes. Follow its exact leaflet; do not copy the gdex timetable across.
eSHa does not recommend combining ndx with other eSHa products. A comparison page is not permission to add both bottles or to switch between them at an arbitrary interval.
Sources
Match the actual bottle and livestock to the instructions. Neither a correctly scaled amount nor an anecdote establishes universal safety.
Written by Richard James
Aquarist, author, and creator of ShrimpKeeper.co.uk. Helping hobbyists achieve professional results through precision dosing.
View My YouTube Channel & Books →While eSHa gdex is generally considered safe for shrimp and snails at standard dosages, eSHa ndx is NOT safe for most snails (particularly Nerite and Mystery snails). If you have prized invertebrates, it is best to treat fish in a separate quarantine/hospital tank.
The Summary: Which one do I buy?
If your fish is gasping, flicking, or has localized skin patches, get eSHa gdex. If your fish is thinning out despite eating, or if you see worms coming from the anus, get eSHa ndx.
If you're unsure, many experts suggest starting with gdex as it is the "gentler" of the two treatments and addresses more externally visible symptoms.
Calculate Your Parasite Treatment
Use our calculators to get the exact dose for gdex or ndx based on your aquarium volume.