Inherited Tank From Friend And Need Advice

So I got a setup tank from my friend because tbh he had just stopped looking after it. I have my own marine tank, and only just getting back into that, but know nothing about freshwater. Tank is small, too small for couple of residents I think but I will need to address that later. I reckon it's only about a 40ltr tank, maybe 60 tops. In it had no plants, one red tailed shark (between 3-4inch), 2 neon tetras, and a blue lobster (about 5inch maybe bigger :/). I know tank size is small but taking them and waiting til I can afford bigger tank was better than leaving them and letting them slowly die.
Had it bout 2 weeks and had cleaned it out over time (sand was black and water was pretty much black when I got it!!). Fish looked happy then one day a tetra went missing. Thot it was cruel having one tetra on its own so I went out and bought another 6 (wasn't going to add any fish at all til one died, but I then read tetras meant to be in larger groups somewhere). So that was couple of days ago. Next morning my 7 tetras, was 6. Never morning again 6 became 5. This morning 5 has become 4. What's going on? I assume I'm not finding them cos they are being eaten but are they just dying or is the lobster killing them?
I don't yet have any freshwater test kit (I'm on it!) but one mistake I did make was a large water change. I used purely ro water I realised my mistake next night but it was only about 60-70% change so still some tap water in tank. I also then added some 'aquarium salt' to try get minerals back into. This was night before I added new fish so not convinced that is problem. Help :/

Thanks for your reply. Yeah the salt (api freshwater salt crystals) was only added as I realised ro water in freshwater was a terrible idea (I forgot all minerals in marine were added back in with salt). It was just habit using ro water.

I'll look up water quality, never needed to know as use ro for marine. Would never have added more tetras if I thot they would be in danger (the other 2 seemed quite peaceful with tank mates for a couple of weeks )

Do you think tetras are dying themselves or is the shark/lobster killing them?

Pretty irrelevant (hopefully there not on menu too) but also 3 small ghost shrimp (best guess) in tank.

Thanks again for help

It is quite possible that the tank was suffering from old tank syndrome. This happens when a tank is neglected and few water changes are done. The fish use up minerals in the water and they secrete organic chemicals, which build up. They also excrete ammonia, which is turned into nitrate and that also builds up due to lack of water changes. The nitrate and some of the organics are acidic so any carbonate in the water is used up and the pH falls. Tank water will evaporate and if the previous owner was in the habit of topping the tank up rather than do water changes, this increases the mineral content, especially calcium, which makes the tank water harder and harder.
The result of all this is that the nature of the water in the tank becomes very different from the water that comes out of the tap and a big water change can harm the fish. The tank water was also likely to be quite different even after the big water change compared to the shop tank. And using RO water would have changed the water even more as that has no mineral content, I'm afraid.

Neon tetras are not very salt tolerant and as you put that in before you got the new fish, they would not have been very happy when they got into your tank, I'm afraid. The API salt is just an expensive version of the salt you put on your food, plain sodium chloride.


You can use RO water for freshwater fish, but like with marine fish you need to add the right kind of minerals back in. If you want to use RO for this tank, you will need a GH and KH tester in addition to ammonia, nitrite, nitrate and pH. The first think is to find out the prefered GH and pH of the fish you want to keep then add remineralisation salts to each bucket of new water at the dose rate to get the GH and pH you want. Without the testers you will be flying blind. Remineralisation salts - Tropic marin or Kent RO right (liquid or solid)


Anything with claws will try to catch fish, and the bigger the clawed creature is the more successful it will be. The lobster (possibly a crayfish of some sort) does need to go. The shark needs a bigger tank, so either rehome that or buy a new tank.

We all make mistakes. Fish keeping has a huge learning curve. I know you already have marine fish, but you now have another set of things to learn about fresh water. Ask any questions, no matter how trivial. Someone on here will have had the same problem at some time