Every time you send or exchange ERC-20 tokens, you have to pay network fees in ETH. These charges are also known as GAS or GWEI.
On the Ethereum blockchain, network fees are billed in GAS. GAS measures the computational power required to process transactions. For miners, this is a measure of transaction value.
Transactions automatically convert ETH billed as network fees into GAS. The transfer amount does not affect the network fee amount. It's all about the current congestion of the network.
In the following screenshot you can see the ERC20 ETH token and the network fee written in ETH.
To replenish your ETH balance, exchange any coin for ETH in the "Exchange" tab. Alternatively, you can send ETH to your Erium wallet address from another platform.