What are some things that my green spotted puffers could play with?

I have two green spotted puffers, they are small and live together in a 10 gallon tank. I have no problem with them in there, they are still young so I have not turned it into a brackish tank yet. But any tips about that i would appreciate. I was wondering if there are any household items that I could use as a toy to keep them occupied. I have a large piece of dead coral with bubbles underneath and a forest of plants around it. in the middle i have a thing with holes for them to swim and hide in. resting against that i have a large shell that i made into a tunnel so that they can hide and swim through there. on the other side of the tank i have 2 more plants ( not next to eachother) and some open space for them to swim. there is gravel on the floor and i have 2 lights. I was also wondering, how long should i keep the lights on each day, and how do i add/change the water, without making the ph level go down. also is it good to feed them algae gel and baby brine shrimp? oh, i also have a small shell in the gravel upside down for them to sleep in. also, how do i keep the water from evaporating so quickly? thanks in advance