Thusly FishNoob Spake Unto All:

PH is between 5.0 and 6.0 (bit closer to 6), ammonia is 0 and nitrite
is 0.1.

That's a low pH. Unless you are breeding sof****er fish like neons,
you'd have greater margin of safety if you put a couple of limestones
in the tank (which'll buffer the water and give a pH of 7.5).

So it's okay to put some limestone in? I read some websites that said
that rocks were okay to add as long as they *weren't* limestone

Yeah, that's because most websites don't know anything at all about
water chemistry, and simply quote eachother and/or books by authors
who don't know anything about water chemistry either, therby
perpetuating a whole range of myths.
Limestone in water will neutralize acid, and raise pH. Most of our
fishes come from water with a pH of about 6 - 8. This much everyone
know. What the websites etc miss is that the dissolution of calcium
carbonate has an endpoint at 8.3 (which, incidentally, is a 100% safe
pH), and that the process speed decreases the closer you get to 8.3,
so the actual pH in an aquarium filled with limestone will stabilize
around 7.5. That is, you _can not_ kill your fish by raising pH with
limestone. You will never reach dangerously high pH's. Ever.

Then there's the issue of "the right pH". The importance of, and the
sensitivity of fish to, pH is monstrously exaggerated in aquaristics.
Fact of the matter is, pH 7.5 is _perfect_ for every freshwater fish
on this earth EXCEPT if you're breeding (and I do mean breeding, not
just keeping) blackwater fish like neons or if you're a high-tech
plant aquarist who want maximum amount of free CO2 in the water. The
reason you may need to lower pH when breeding blackwater fish is
because the fish may use low pH as a trigger for spawning, or the egg
membranes may become impermeable to the sperm. The adult fish' health
is completely unaffected.

Every fish on earth, including the marine ones, will also greatly
prefer a stable pH over a fluctuating one, and the limestone will make
the pH stable.

In your case, with your very low pH, a limestone (or shells etc) may
actually be a lifesaver for your fish.

I also read somewhere (thought it was here, but can't find it here
now) that placing some seashells in the tank will increase the ph. I

Yes, anything that's made of calcium carbonate: shells, fossils,
limestone, chalk, even eggshells, but NOT blackboard chalk (that's
calcium SULPHATE, aka gypsum, and will not buffer pH at all).