Stellar has announced it will doing a reset/cleaning of their testnet to the genesis block, once per quarter to keep down spam, for general efficiency, and maintenance best practices. The first reset is scheduled for January 30th, 2019.  Be aware and plan accordingly.

 https://www.stellar.org/developers/guides/conctcei

Periodic Reset of Testnet Data

In order to preserve a good experience for developers, the SDF testnet is periodically reset to the genesis (initial) ledger. This ensures the removal of spam, minimizing the time to catch up to the latest ledger, and easier maintenance of the system over time.

When this happens, all ledger entries (such as accounts, trustlines, offers, etc), transactions, and historical data are cleared for both Stellar Core and Horizon.

As a result, developers should not rely on the existence of any accounts, or the state of any balances within testnet.

Starting in January 2019, the testnet will be reset once per quarter (every three months):

• January
• April
• July
• October

SDF will announce the exact date with at least two weeks notice on the Stellar Dashboard, along with several of Stellar’s online developer communities.