Well with not a big budget, I'd suggest staying to a smaller size tank. Also, I assume with the budget restraint that you'll want to do a fish only system and not a reef? I have a 40 breeder predator tank. I really like the size and it would be very affordable since you can buy the aquarium for $40 at Petco during their $1 per gallon sale. I also run a 20 long sump that I picked up for $20 during Petco's sale. If you want to stay fish only, then you wouldn't need expensive lighting or dosing equipment or anything like that. You'd just need good filtration - definitely a skimmer and probably a media reactor too. In terms of predators, the dwarf lionfish are a great option for smaller tanks. They're not too expensive to purchase either. They have great personalities and are a lot of fun.