Depending on where your water is sourced from, it may not necessarily be your media that's giving you grief. Where are you getting your water from a well system or municipal water supply?
I don't know anything about Nicaragua, especially when it comes to sourcing fishtank products, but if you can source a KH test kit (API is a popular one and usually sell for less than $10 here in the States), it may clear up a few things. KH (carbonate hardness) is a measure of carbonates and bicarbonates in your water. Carbonates act as a sort of pH buffer, and if there happen to be any in your source water, than they can cause the same symptoms you've been describing, where the acid temporarily drops the water pH, only to be then neutralized by carbonates, causing the pH to swing back up to where it was originally.
In my own personal experience, well water can be notoriously high in carbonates. For example, the well water we have at my place tests at a 8pH and 18dKH and takes 4-5 cups of muriatic acid to bring one 275 gallon IBC tote to a pH of 6.5.
Carbonates aren't super difficult to deal with, but it helps to know about them as a factor so you aren't pulling your hair out
Here are a couple tips:
1) Buy that KH test kit if it's available and reasonably priced. It's a lot less stressful when you can see those levels slowly climbing down, plus you'll know when to ease up on the acid so you don't crash your pH once the carbonates run out
2) Use cheap muriatic acid for bulk adjusting. Ever try eliminating carbonates with phosphoric acid? Don't. Your wallet will thank you.
3) Buy a separate container to treat water in. You're going to need to replace water in your system due to evaporation and such, and it's a lot less stressful to adjust water that doesn't have fish in it.
Hopefully some of that was helpful. I'm sleepy and I tend to ramble