You can keep cherry barbs and endlers together. Cherry barbs are schooling fish and need a group of six or more.
I'm a little unsure of your final stocking though, so I don't know if this would be overstocking the tank.
Corys should be in a school of at least six. Keeping four of one kind and four of another doesn't count as one school but two, which means that you'd have two too small schools.
My advice would be to pick one kind and get six or eight of those.
Is there any ammonia in your tap water?
If not, adding just water wont do anything. When you started adding the fish food did the ammonia ever go above .50?
Prime doesn't hurt anything, it detoxifies ammonia and nitrite along with chemicals such as chlorine and chloramine.
It doesn't remove...
How did you cycle the tank?
It looks to never have finished the cycle and whatever the ammonia source was it has been very little. I wouldn't cycle a tank with such low numbers for ammonia, simply cause the bioload it's going to be able to handle is then tiny.
A tank is only cycled when it...
Well, if that's the question I'd say that technically yes, it's somewhat a waste of money and product to change the water if it isn't necessary, say if parameters are 0 ammonia, 0 nitrite and 10 nitrates.
However, there are a bunch of things in the water that we don't measure such as...
IMO 16 gallons is too small for corys.
Some sites state 10 gallons as minimum, some say 20 and others say 30 gallons.
If I were you I'd re-home them as they should really be in groups of at least six. Once you upgrade to a 30 or 40 gallon you could definitely get a school of them, though.
In a cycled tank there should always be 0 ammonia, 0 nitrite and some nitrates. 20 nitrates or below (but rarely 0 unless the tank is very lightly stocked or densely planted) is considered a good range. In a tank that's well filtered and appropriately stocked, nitrates should never reach more...
Quick start is made for cycling a tank faster. Technically it could help by growing some beneficial bacteria that might help getting rid of the possible ammonia and nitrite faster.
Prime detoxifies ammonia and nitrite while removing chlorine and chloramine. The idea is to detoxify whatever...
Some more information on the tank would be very helpful.
What size tank is it?
What fish live in it?
How long has it been running?
What are the exact water parameters?
What do you use to test the water parameters?
The snail and the driftwood doesn't have anything to do with it all, I think. The snail dying could be a coincidence and the white stuff on the driftwood is completely normal. When you add driftwood to the tank and it hasn't been soaked before it sometimes grows this white slime which...
To me buying a reliable test kit is never going to be a waste or unnecessary.
I believe API liquid test kit is the best one, hence that's what I'm going to suggest to anyone. It's people's own choice what they buy, but I'm here to give the best possible advice I can.
IMO test strips are so...
Water changes can never hurt. Assuming that there are toxins in the water, your fish will appreciate the clean water.
You can rinse your filter in tank water, just don't be too thorough.
If you're gonna buy a test kit, definitely buy a liquid one.
The vast majority of the beneficial bacteria lives on your filter media. This is why when people say that they transferred some decorations and plants to a new tank to "help it cycle", they're told it doesn't matter. There is beneficial bacteria on all surfaces on the inside of the tank, but...
I don't know if you're overstocked, but I can tell you why your water parameters are going crazy.
When you toss your filter and swap it for a new one, you throw your cycle out. All the beneficial bacteria lives in the filter material and without it you're back to 0, meaning you'll have to cycle...
What's possible is that since he didn't eat the food the water parameters turned bad and left your fish in toxic water. I would definitely not trust any pet store to test my water since they seem to mislead people all the time.
Do you have a test kit of your own? If not I suggest you buy one...