My sand was arag alive... the kind that is wet/sea water...
for rock... that alone will kill your budget...
If you are lucky enough to know someone that could give you a piece.
Mine came from ARC reef store.. it’s a conservation center/Miami...
it’s the real thing... it was a lot (IMO)
But, I have bigger tanks/ not, huge ... 75g and 100g. But, 1 piece was between $150-$250. I’m just being honest...
For filtration sand/live rock help (possibly) no filter. But, since you have the filter... clean with vinegar, buy new media... and, go bare bottom or sand, NOT gravel. No matter what... you will have to let your tank cycle. The sand is a good start (it’s $20-$25 for a bag/wet kind)
I would wait on rock (only because of tight budget)
I can’t help on changing from freshwater to saltwater/ I’ve only had salt/marine/reef tanks. I know you would need to clean it well... definitely, get advice (on here) or from people who have experience, and (successfully) switched. Sand vs bare bottom has pros and cons for both. Sand (live/wet kind) can help with biological filtration, some people think bare is easier to clean. Or, personal preference. Do, let your tank cycle before adding livestock. 4-6 wks. It has to go through that and the ugly phases of algae blooms. It’s just part of it. But, it could buy you some time.. to get supplies, equipment, lights, etc. Also, I might should have said this earlier.... I don’t want to discourage you. I love this hobby. You may want to research everything and then decide whether you want to do this. It’s not easy....IMO... it’s expensive and lots of learning, setbacks, along the way. It’s a lot of work, sometimes, it’s like cracking a hidden code kinda’ hard. On the other hand, when it finally goes right....healthy/disease free, beautiful fish. Or, your baby frag starts to grow (you celebrate 2 polyps!) And, you get hooked! That’s so awesome! I’m just saying.... it’s not for everyone... go slow... if nothing sidelines you... it could be the hobby of your dreams. It’s ok to take some time to decide, especially, before you break the bank. Lol Good luck! If I can do this, anything is possible!