Why are my new fish dying?

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Are you struggling to keep new fish alive in your tank? One of our readers asks our experts why this is happening...

Q) I’ve been keeping fish for over 18 months and for the last eight months I’ve been running a Fluval 123 l. The tank is quite empty with just two plecos and a few recent babies, three tetras and a Dwarf gourami, plus a couple of shrimp. But I’m struggling to add new fish. Three Dwarf gouramis died after only a few weeks. I even upgraded the filtration system to the 'in-tank' media baskets for higher quality filtration, using chemi-pure Green and Biogravel in them along with foam. I do small weekly water changes with RO water and I run an Eheim skim 350. The tank is planted and also has slate and a big piece of bogwood. The tests on water seem good and I only add fish after research on suitability for my set-up. The fish already in there seem to be very happy, but every attempt at adding new fish results in them perishing. Am I missing something really obvious? Any advice wouId be very welcome.

ANDREW, VIA EMAIL

A) Neale replies: My mantra with community tanks is to choose species adapted to your water conditions and then select from those species you know have a good track record.

You say you’re using RO water, but I’d just like to check something here: are you using just RO water and nothing else? By itself, RO water isn’t suitable for most fishkeeping. It needs to have at least some minerals added back.

Crucially, it has no ability to resist pH changes, while great lumps of bogwood will release acidic chemicals that could cause the pH to drop quite quickly.
If you’re keeping a mixed community of South American and Southeast Asian species, then mixing some hard tap water with the RO at a 50:50 ratio should do the job nicely.

But when it comes to choosing fish, there are some species that, frankly, are hit-or-miss for a variety of reasons. Often, the problem is that they’ve been bred to a price rather than for quality. While there are good examples of these fish out there, particularly if you can find locally-bred or wild-caught specimens, I’d personally avoid Neons, Guppies, Dwarf gouramis, and Ram cichlids. Or at least, if you did want these, you’d look for the best quality ones you could get — rejecting any from tanks with ailing specimens — and then quarantine them ruthlessly before adding them to the display tank.

With Dwarf gouramis in particular, the problem is disease, with particularly pernicious viral and bacterial diseases of various sorts seemingly near-ubiquitous. I dare say that if quarantined and maintained in optimal conditions these fish can do well, but for casual fishkeepers, they are probably best avoided.

I’d hesitate to recommend fish without knowing your precise water chemistry. But there are some species with broad tolerances that generally adapt well to most conditions.

A personal favourite is the X-ray tetra, Pristella maxillaris, a species that is both pretty and hardy. Able to handle even ‘liquid rock’, it’s one of the most versatile South American tetras out there and gets along with other community fish. The Cherry barb, Puntius titteya, is another colourful, peaceful species that adapts to a broad range of conditions. In a planted tank they are especially charming, their sexually dimorphic colours — with the red males and the peachy females -— really standing out.

Or perhaps you could try a group of Corydoras catfish. Many of the most popular species should be fine.