can you add more sand to an established tank?

Of course you can, especially as sand is inert. You might want to rinse it first. Then there are several ways to do it without too much mess.

Turn off your filter / powerheads, just in case. Use a length of hose (5/8" works), position one end just over the spot you want to fill, keeping the other end above water. Add sand using a funnel. 2 sets of hands are usefull.

One set of hands method, that I saw on TPT, is to fill a 2 litters soda bottle with sand, close the top with a finger, insert the bottle unto the tank over the spot, use your finger as a gate.

v3

Sure you can - no problem - just be sure the sand (whatever sand it is you're using) is well rinsed beforehand.
And OVT's method of introducing it sounds A-OK.

OVT said:

One set of hands method, that I saw on TPT, is to fill a 2 litters soda bottle with sand, close the top with a finger, insert the bottle unto the tank over the spot, use your finger as a gate.

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This is a great method and not much mess

I guess it depends on how much you are adding. If it's only a small amount it should be easy. If you are adding a lot, be careful.

When you rinse the sand you're probably just using tap water, so be sure to add enough dechlorinator for the entire volume of the tank. Also, if you are adding a lot of sand, watch your tank temp. Adding a lot of sand, rinsed in cold water can drop your temp quickly.

Put the sand in a one gallon zip lock. Put the sandbag on the substrate with the ziplock end down. SLOWLY open the bag. SLOWLY lift up on the bag as the sand comes out of the bottom.

all great ideas.. thanks guys!