Very nice DIY work. To increase salinity level, u can add more salt to your di water when making your saltwater. Since its a week only, your ammonia hasn't reach a spike thus reading is zero. It takes time for nitrifying bacteria to grow to decompose your market prawn. Continue adding bacteria to increase the population. U should also test your nitrite now instead of nitrate cos your bacteria level should be low now to breakdown nitrite to nitrate. Once ammonia has reached it's peak, it will dropped and nitrite level will rise. Once nitrite starts to drop, cycling will almost be complete. It's takes about a month or so to fully cycle your tank.