Hi and welcome to the forum
It depends on the fish you keep. Shells and limestone/ sandstone rocks found at the beach are calcium based and if you have enough of them in the tank, they can increase the pH of the water. If you have fish that like a pH above 7.0, then that is fine. However, if you have fish that come from acid water (pH below 7.0), then don't add shells or limestone/ sandstone.
Some beaches have brown or black pebbles/ stones and these are usually granite and do not change the pH. These can be used in aquariums assuming they are free of organisms like shellfish, barnacles, algae. And as long as they are free of chemicals and pollutants like fuel from boats.
A way to test rocks for calcium is to pour some white vinegar on the rock. If it fizzes and bubbles, then there is a reaction between the vinegar and calcium and that rock will raise the pH. Rinse the vinegar off before using the rock in an aquarium or pond.
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Re: cycling your tank in 2 weeks. Did you add any ammonia or filter bacteria to get the filters going?
Most tanks take around 4-6 weeks to cycle and you need to add a source of ammonia for the filter bacteria to feed on. If you didn't do this, then you might be doing a fish in cycle, whereby the fish are in the tank while the filters develop the good bacteria. If this is the case, just reduce feeding to 2-3 times a week and do a 75% water change and gravel clean the substrate 4-8 hours after feeding. In a month's time you can increase feeding to every day and do the water change and gravel clean once a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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Did the shop tell you how to clean the filter?
Does the filter need to have filter pads/ cartridges replaced monthly?
If yes, do not change them unless they are falling apart because you get rid of the good filter bacteria. Just wash the filter media in a bucket of tank water and re-use the media. tip the bucket of dirty water on the lawn.
If you have filter pads that contain Ammogon/ Zeolite (white granules), you can tip the Ammogon out and throw it away, and don't replace it because it stops the filter bacteria developing.
If you have carbon (black granulated substance) in a filter pad, you can chuck the carbon away and re-use the pad. Don't replace the carbon because it is not necessary.
