Have you tried a reef with coral and inverts? With any aquatic system, the same basics apply. Nitrates must be kept under control. You were under the impression that nitrates are harmless. Now that you know otherwise, take what you've learned and apply it to the next. High nitrates accumilate when there's a lot of waste and lions and eels and other bulky fish push out more waste than most other fish. Even freshwater fish like oscars and other cichlids as well as goldfish are notorious heavy waste producers. Saltwater systems, even fish only benefit from protein skimmers to help control nitrates. Please note: protein skimmers do not work with FW
To convert, you'd have to tear down and reset using FW
materials like gravel and FW
rock if you plan on having anything FW
with one exception...African cichlids. If you choose to set up for African rift lake cichlids, you can keep the rock and substrate. Though the live of the rock would transform to suit it's new environment, it can still serve it's purpose without mineral conflict. Live rock is high in mineral/metal content like calcium and would kill most other FW
fish, but African rift lake cichlids are natural to the higher levels. You'd just need to change out the water and replace with all FW
A trick to good natural nitrate control is building a DSB
(deep sandbed). This includes at least 3" or 4" of substrate and animals that crawl through the sandbed. Loaches are good in freshwater systems. Wrasses, eels, bristleworms, snails etc are good for marine systems. The thick layer of substrate chokes the bottom layer of oxygen, making an ideal envrionment to cultivate denitrifying bacteria which would eat up the nitrates. The animals in the DSB
release hydrogen gases that accumilate during the denitrification process. Those gases then dissipate in the atmosphere, completing a full nitrogen cycle. This method of nitrate control is preferred in systems that require a lot of rock work such as reefs and African rift lake cichlid tanks. It eliminates the need for vacuuming the substrate, though you still need to exchange water.
Sorry to hear about the lion. Did you know that coral, though may be more expensive to support with most, they are much easier to keep alive than fish.