Gregson-Clark sells the Eco-505. It's a secondary tank that hooks to your primary through an internal hose WITHIN your hose. It hooks to your chemlawn gun with a second trigger - allowing you to spot spray a chemical (weed killer or insecticide, etc)... I am considering it. Or creating something similar (because the price tag on this little thing is absolutely insane). Just looking for feedback from anyone who has used/owns one. Thanks!