Fish Dying

A couple of things spring to mind.

You say your ammonia reading is 0.25, but you don't give a nitrite reading. 0.25 ammonia is right at the top end of safe for fish, and it worse at high pH like yours. You could do with getting it down with water changes. And without a reading for nitrite it is impossible to say if it is high enough to affect the fish. Here too, 0.25 is the highest it should be allowed to get.
How long was the tank without fish for? If it was longer than a few days, the bacteria in the filter would be affected. They wouldn't die as quickly as we used to think without a food source (ammonia made by the fish for the first type of bacteria and the nitrite made by the first ones for the second type) but is known that they become dormant with insufficent food, and the longer they are without, the longer it takes them to recover. The nitrite eating bacteria are more delicate than the ammonia eating bacteria and would take lober to recover form the tank being without fish, so you may have a reading over 0.25 for that.

Cichlids from the Rift Lakes like very hard alkaline water. The tufa rock dissolves slowly, increasing both hardness and pH. The soft water fish that you have (neons and dwarf gouramis) would not like this, but the guppies and platies should be OK. What pH is your tapwater - leave a glass to stand for 24 hours and test that as well as it may change on standing.

The baby guppies - they have been eaten, I'm afraid. That's what happens to fry, it's why fish have so many. If you have a lot of fine leaved plants, some may make it. But being eaten is not necessarily a bad thing or even a 190 litre tank tank would quickly become over populated

If there was some nitrite in the water, however low the shop meant by negligible, it does suggest that some bacteria died off while the tank was without fish. You need to keep the ammonia and nitrite levels as near zero as possible with water changes. [quick question - you do know about what the filter bacteria do?]

Are you adding a pH lowering product to your water change water? That won't do any good I'm afraid with the carbonate that is in the tank. And messing with the pH by adding stuff from a bottle is not a good idea in general.
Let me explain. You have soft tap water. That means there is very little calcium and magnesium in it. And probably very little carbonate as the two usually go together. Soft water is usually acidic, that is the pH is below 7. Carbonate is a buffer; it prevent changes in pH, ie it buffers the pH. The water in the container may drop through adding a pH lowering product. Your tap water is soft and won't have much carbonate to buffer the pH. But when you add this water to the tank with its high carbonate, that will buffer the tank's pH against change and the new low pH water will have no effect on the tank's pH.

The African cichlids your son had sound like rift lake cichlids which need very hard water and a pH well over 7 (more like 8 ). He would have added things to the tank to make your soft tap water hard by using decor made of calcium carbonate (the tufa rock, the oyster shells despite what the shop said). Calcium carbonate dissolves slowly adding both calcium and carbonate to the water and increasing the pH. He would probably also have been adding some special salts, usually called Rift Lake salts, to the water, either ones he bought or a mixture he made himself.
Rift Lake cichlids need a very peculiar water chemistry. The water in these lakes is not just hard; it contains more weird minerals than hard freshwater usually does. These minerals will affect fish from other lakes and rivers, even those that come from normal hard water like guppies and platies

You need to remove all those calcium carbonate things from the tank, which includes not only the tufa rock but the oyster shells as well. That unfortunately mean replacing the substrate as you won't be able to get all the shell out. The substrate may well also contain crushed coral, more calcium carbonate. You will need to put the fish in a covered bucket sized container (fish can jump!) together with your plants and enough tank water to fill the container then remove all the substrate. Prepare the new substrate before you start, wash it till no more dust comes out, then start catching the fish. Once you have the new substrate in, you can replant and put the fish back. Don't put the water back, use the opportunity to put some fresh water in.
Then you need to do daily small water changes. 25 litres out of 190 litres is 13%. This amount will not stress the fish by adding too much water of different chemistry. But don't add anything to alter the pH to the new water. After a couple of weeks of these small daily water changes you will not only get the tank water to near your tapwater, you will also keep the ammonia and nitrite lower.
Do you know if your water company uses chlorine or chloramine to purify the water? It might be worth a phone call. The reason I ask is that you mention leaving the new water container to stand overnight but you don't mention adding a dechlorinator. Leaving it to stand like this would remove chlorine but not chloramine. Both of these can kill the filter bacteria so they must be removed. Most of use use a dechlorinator as it removes the need to have containers of water standing round the house for 24 hours. It also means that there is no danger when doing an emergency water change.


Once you have the tank water at the same hardness and pH as your tapwater, you can concentrate on the ammonia and nitrite. You will need to continue with these daily water changes until both ammonia and nitrite stay at zero, especially just before a water change. Then you can go to weekly maintenance water changes. Though 25 litres, 13%, might not be enough. I do more than 25 litres on my 125 litre tank. The recommended level for lightly planted (less than 25% substrate with a plant growing out of it), sensibly stocked tanks is 20 to 25% a week.
For pH, you need to check what your tapwater is after it has stood for 24 hours, just a glass of water left to stand overnight. pH usually changes on standing (mine goes up by 0.2) and it's the stood water pH you need to compare your tank to.

Changing from Rift Lake cichlids to 'normal' tropical fish is almost as bad as changing from marine fish to freshwater. It'll take a while to do it as you have to go slow with fish in there. But you will get there. You may find that you can't keep hard water fish in your tap water, but a lot of fish keepers would kill for soft water. there are far more types of fish that can be kept in soft water. Neons in particular need soft acidic water; they are suited to your tapwater rather than the water that is currently in your tank.

OK the last bit first.

Fish make ammonia as their waste; it is their equivalent of urine. This ammonia ends up in the water where it burns the fish's skin and gills - the latter makes it harder for the gills to absorb oxygen. In a cycled tank there are bacteria in the filter which use ammonia as food, and they turn it into nitrite. This is also poisonous, it sticks to the fish's blood cells instead of oxygen so the amount of oxygen getting round the body is reduced. Aonther species of bacteria lives in the filter which uses nitrite as food; they convert it into nitrate. Nitrate is only poisonous at high levels and we remove it with water changes.
The filter in your tank would have grown these bacteria when the cichlids were in there, but without any fish to make the ammonia the bacteria wouldn't have any food. They won't have died quickly, most of them would have become dormant, but it takes a while to come back from this dormancy; the longer they are without food, the longer it takes and the more likely they would die. The nitrite eating bacteria would be affected more than the ammonia eaters as they are more delicate. This is assuming the filter was in water all the time. Dried out filter media will lose bacteria much faster.
So depending exactly how long it was between the cichlids leaving the tank and your fish going in, you could have dead bacteria and/or dormant bacteria which are only coming back to life slowly. Your fish are busy making ammonia but there are insufficent bacteria to eat it all. And if you have some ammonia eaters, you'll get nitrite building up through lack of nitrite eaters.

The good news is that provided the media was kept wet and you haven't washed it in tapwater you should have some bacteria in there. It's just a case of growing some more which is a lot faster than starting with a brand new filter with no bacteria at all. But it will mean doing water changes. Because the chemistry of the tank water is so different from your tap water, you'll need to do lots of small changes rather than a few big ones or the sudden change will harm the fish. Once you've got the tank water to the same as the tap water by these small water changes, bigger water changes will be safe.


So you are using dechlorinator. Good  You can add hot water to your cold to warm it. Is your hot water stored in a cyclider? If it is, boil a kettle. If not and the hot water is made on demand, you can use that. I add about half a litre of boiling water to a 7.5 litre bucket to get the temp right. Dechlorinators work instantly so it can then be used straight away. With three main tanks (and a fourth while my baby shrimps get big enough) I wouldn't be able to let enough water stand overnight 

The 19 litre tank would be fine for the fish for the duration. It shouldn't take long to do the substrate if you get it all ready first. Washing it can be the slow bit.
I am not the best person to advise you on plants. I can't keep most of them alive - I even managed to kill just about all the duckweed in one tank this summer  . I know there are all sorts of plant substrates. I know that 'easy' plants can be grown in plain gravel or sand using root tablets or liquid fertiliser added to the water column (I can manage java fern and anubias with liquid ferts). For this aspect of your tank, can I suggest you start a new thread in the plant section, you'll get more help there from people who  know what they are talking about.