Possible Sick Cory?

Cories do dash to the surface because they need to take gulps of air and then absorb the oxygen in it from the gut. So the dashing to the surface and the death were probably not related.

When the tank has sand on the bottom rather than gravel you can see more clearly the way cories feed. They take a mouthful of sand, sift it for food then pass the sand out of their gills. I have sand with my cories and it is fascinating to watch. I would imagine anything they put into their mouths thinking it's food gets passed out of their gills the same way.


But having said all that, I need to ask what the ammonia and nitrite levels are in the tank. You added quite a lot of fish in one go (it is normally recommended to add one third of the amount of fish already there in one go), and the tank probably hadn't finished cycling after you got the neons. If you do have any ammonia and/or nitrite this will affect all the fish, with the weakest being affected first.
If you don't have your own testing kit, a shop will test a sample of tank water for you but make them give you the numbers, not just something like fine or a bit high. Basically, anything above zero is not fine regardless of what the shop may say.

Once you know your levels, come back and we'll see if you need to do anything.

The problem with the strips is they don't test for ammonia and you need to know what that is. After 4 weeks with neons, you should have grown some bacteria but the new guppies and cories would have increased the ammonia being made by their waste so you may not have quite enough, and you may have ammonia in the water.

What were the actual readings for nitrite and nitrate? As far as I'm concerned the only acceptable reading for nitrite is zero.
Anything above that is not good. Nitrite binds to the fish's red blood cells stopping them absorbing oxygen; it does to fish what carbon monoxide does to us.


How often and how big have your water changes been since you got the first fish?

Test the nitrate level of your tap water. This is the lowest you can get it just by doing water changes. The UK legal limit for nitrate in drinking water is 50ppm and some regions' tapwater is not far short of that.
The aim is to keep your tank nitrate below the tapwater level plus 20. If it is higher than that, there are a few possible reasons -
feeding the fish too much
too many fish in the tank
not enough water changes
water changes not big enough

As a general rule of thumb, a tank that is reasonably well stocked and doesn't have many live plants needs 25% of its water changing every week. Jungle-like tanks can get away with smaller water changes, as can lightly stocked tanks.



Liquid reagent tests (the kind with bottles and test tubes) work out cheapest per test. They cost more to buy but do a lot more tests than the strips.

Glad to hear the water change helped. Regular water readings (perhaps daily for the next few weeks / until things settle down a bit) and regular water changes (weekly, or more often if any ammonia or nitrites or else high nitrates) should hopefully help to maintain the water quality at a good level.

What is your water hardness? Your water company website should be able to give that information as well as your test strips - the exact result and unit of measurement would be really helpful to know. Once the tank has settled, then you'd be better choosing the replacements for the two dead fish of a type that is suitable for your water - eg possibly more guppies if your water is hard or else more neon tetras if your water is soft.

No.
They mess with the water chemistry. When I tested the water in my Brita jug the pH had dropped from 7.5 to off the bottom of the scale - somewhere below 6.0. They swap calcium and magnesium ions for hydrogen ions, and pH is an upside down measure of the amount of hydrogen ions - the more there are the lower the pH. You need something that removes minerals and doesn't replace then with hydrogen ions.

If you want to 'soften' your water the better option is by using RO (reverse osmosis) water. This is water that has had all its minerals removed. Fish can't live in pure RO so you either mix it with your tap water to 'dilute' the hardness or add remineralisation salts to put some minerals back, just not as much as in your tap water. The problem with this is that you have to use the same ratio of tap/RO at every single water change or add the right amount of salts to the new water at every single water change. You can never use pure tap water again.


With hard water like this, the guppies should do better than the neons and cories. In your other thread you mention that the guppies are having problems which is not surprising with their reputation these days. With a 50 litre tank and hard water, look at replacing them with endlers livebearers instead. Despite all the ones in shops being endler-guppy hybrids, they are more robust than guppies and in my opinion prettier. The males don't fight like guppies, another plus point.