Acrylic or glass - Doesn't matter.
Neither material will be causing your fish to die.
You said you do not have a water test kit - so therefore you will be unaware of your water readings (
ammonia,
nitrate,
nitrite and
pH)
Poor water will kill your fish - get a water test kit - or if you can't get one take a sample of ytour water to a pet store (they will often check it for you for a very small charge if not free)
You will most likely have high readings for the above mentioned toxins in your tank and even in small quantities these toxins are harmful for fish.
How often do you change water?
Your Aqu' info says you don't know about the
nitrogen cycle?
With your current stock you are most likely overstocked (which might mean your filter may struggle to keep up with the bio load[ammount of waste prduced] you say you have "way too many" guppy babies... How many?)
Do you have a heated & filtered tank?
you said 6.5 gal tank, your Aqu' info says 10gal?
.
Oh and you need to get a heater for the tiger barb soon too or re introduce him to the other tank (which I hope is heated)
An unheated tank is not ideal and though he may look fine now, he will most likely get sick.
If you do nothing else, then at least read this:
http://www.fishlore.com/NitrogenCycle.htm 