Check and monitor your water quality (ammonia, nitrite, nitrates) using a liquid-based test kit. You need to ensure that water quality is consistently 0 for each of ammonia and nitrite, and no more than 20 above tap water levels for nitrates.

Water quality needs to remain consistently at this level, even just prior to your scheduled water change (weekly presumably), to prevent fish succumbing to illness or dying - if any of those levels are higher, then do more frequent and/or larger water changes.

What an absolutely devastating event to have occurred.

I would have thought that if it was this product per se which killed your fish, then others would have experienced the same and the products would no longer exist. One possibility would have been that something was awry in the tank already (detectable or not) but, for whatever reason, this was “the straw that broke the camel’s back”. *However*, I’ve just had a look on Amazon and have seen your review and a number of others’ reviews which have had similar consequences, which collectively are concerning, so thanks for drawing attention to this as it might save others from the same fate! Always worth checking the worst reviews of any product to see what the nature of those reviews is.

A quick google search brought up this article https://www.practicalfishkeeping.co.uk ... les/10-ways-to-beat-algae which states “There are fewer algae treatments nowadays as UK legislation prevents some being sold, but remember to always check that your water is well oxygenated and filtered to cope with a sudden algae die-off.”

Going forwards, you mention that you “set up a fresh tank” – just checking that you didn’t discard the filter media altogether with its build-up of beneficial bacteria which helps process fish waste? If so, you’ll be going through a cycling process at the moment – this may account for the additional fish deaths but you’ll need to be extremely careful during this time and not add any stock. Read this: https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm

If these 5 fish are still alive, and you want to restock your tank ultimately, and you have a spare filter and tank/container, then I’d strongly advise doing a fishless cycle on the spare filter in its own tank/container to prepare it for the new fish https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm, then, once that’s complete and you’re ready to get new stock, transfer the filter across into the main tank along with adding the new fish. Even if you still had the existing filter media in the filter and hadn’t discarded it, the aforementioned suggestion would be a better way to restock given the recent ‘upheaval’ in the tank.

Best of luck and thanks for highlighting your horrible experience and the existence of other reviews of similar experiences.

This isn't sounding too good. I'd ensure that water quality is in absolute tip-top condition by doing daily water changes, dosing the Kanaplex after a water change. It can get confusing trying to compensate for medication lost in water changes, so I wouldn't try to do that, but just dose as per instructions.

Ensure the filter is providing as much oxygen and surface agitation as possible. If you have an airstone, use it too.

I'm not sure that there's much more to suggest... Are any of them bloated, do you think? (I appreciate you may not know what they're normally like as you've only had them a few days.)

I don't have an ingredients list for Easylife's Voogle. It's not something I would personally risk but you may feel more prepared than me to take that risk.

Could the betta have injured itself on any decor, caught its fin or tail in the filter or burned itself on a heater?

Keep monitoring the water quality - daily or every other day - doing extra/large water changes if required to ensure that it remains at those optimum levels. The 'problem' may resolve in its own time although bettas do undergo changes in colour.

If his behaviour or appetite changes, then that would be the time to concern yourself and we'll try to help further.

You've taken the correct course of action, for the correct reasoning, with regards to the filter media pack. The other option you could do, and which I did with a previous filter like you describe, is to convert from a filter media pack to a purely sponge/foam filter which won't need replacing at all. You could do this by placing pieces of foam in any spare space - in fact, if the new filter media pack hasn't been in very long to start building up beneficial bacteria, you could discard it, keep the old one in place with its beneficial bacteria and pack foam in the spare spaces. After a month or so, by which time beneficial bacteria will have built up on the foam, remove the filter pack and replace it with more foam. During the month or so that the new/additional foam is building up its beneficial bacteria, don't add any more stock.

A film on the water surface is often caused by oils from fish foods and possibly from other products used in the tank.

I would recommend the Kanaplex rather than the Voogle. I have used Voogle for more minor issues but what you describe sounds like a bacterial infection which Kanaplex seems to be for, so I'd definitely opt for it to give the fish the best possible chance. (I haven't personally used it but have used eSHa 2000 and Waterlife Myxazin. However, Kanaplex seems to be for the same conditions.)

I'm seeing a black mark on the tail fin plus foliage/planting - but maybe someone else will be able to see better what you're referring to. In the meantime, what are your current results for ammonia, nitrite and nitrates? The reason for the question is that this may help work out what the black mark is on the tail fin.

It may be worth placing an advert on www.aquarist-classifieds.co.uk too. Good luck.

If you can let us know what your water hardness is, either in GH and KH from your water testing kits or else from inputting your postcode into your water/utility website to get water hardness results in CaCO3 or German degrees hardness, that would be really useful for us to help you choose suitable fish whose water requirements match those of your own water.

[I see, from a previous post of yours, that you were asking about Tetra Safestart .v. other products. What approach did you use for the cycling process ultimately and how have you found it?]