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.