referring to fig1 and 2 , a float actuated water level control valve 10 in accordance with the invention is well - suited for use in an aquarium tank 12 having an overflow wall 14 . the overflow wall 14 separates the aquarium tank 12 into a main aquarium region 16 and a filter intake region 18 . water in the filter intake region 18 flows into the valve 10 , arrow 22 , and by gravity flows downward through a drain 26 , arrow 24 . the drain 26 is mounted through the bottom 28 of the aquarium tank 12 in the filter intake region 18 . from the aquarium drain 26 , water flows through line 30 to a wet / dry filter 32 which includes a sump 34 . a return pump 36 pumps filtered water from the sump 34 through a return line 38 to the aquarium 12 . a return tube 40 is mounted through the bottom 28 of the aquarium 12 in a filter intake region 18 of the aquarium tank 12 . the return tube 40 is generally l - shaped . the return tube 40 has an inlet 42 that receives filtered water pumped through return line 38 , and has an outlet 44 that is positioned to discharge water into the main aquarium region 16 of the aquarium tank 12 , as depicted by arrow 46 . water in the main aquarium region 16 spills over a top edge 20 of the overflow wall 14 into the filter intake region 18 as depicted by arrow 48 . for purposes of illustration , the water level within the filter intake region 18 is shown as a distance &# 34 ; d &# 34 ; below the top edge 20 of the overflow wall 14 . one purpose of the float actuated water level control valve 10 is to maintain water level within the filter intake region 18 at a relatively constant level when the overflow filter system is operating in steady state ( the distance &# 34 ; d is the distance between the top edge 20 of the overflow wall 14 and the relatively constant water level in the filter intake region 18 ). referring now to fig3 the preferred float actuated water level control valve 10 includes a float 50 and a sliding valve tube 52 depending vertically downward from the float 50 . the float 50 and the sliding valve tube 52 are preferably integral components made of polyurethane molded via a plastic blow molding process . the valve 10 includes a vertical valve guide 54 that is mounted on the drain 26 . in the preferred embodiment of the invention , the drain 26 comprises a vertical standpipe 56 having a threaded bottom fitting 57 that is mounted through the bottom 28 of the aquarium tank 12 using an o - ring 58 and a sealing nut 60 . the standpipe 56 has an upper inlet portion 62 that is cylindrically - shaped with a vertical cylinder axis . the upper inlet portion 62 of the standpipe 56 is sized and configured to securely support the vertical valve guide 54 , although the vertical valve guide may be removable from the standpipe 56 . the upper inlet portion 62 is preferably integral or attached to the standpipe 56 . referring briefly to fig4 and 5 , these figures show the upper inlet portion 62 being attached to the standpipe 56 . a support ridge 64 extends inward from the inside wall of the upper inlet portion 62 , and the upper inlet portion 62 preferably fits tightly over the standpipe 56 so that a top edge 66 of the standpipe 56 abuts the support ridge 64 . the valve guide 54 preferably has the same outer diameter as the outer diameter of the main portion of the standpipe 56 . the valve guide 54 therefore fits snugly within the upper inlet portion 62 above the support ridge 64 . the snug fit maintains the valve guide 54 in a vertical orientation . referring again to fig3 the preferred valve guide 54 is a conventional filter core having at least one , and preferably , several openings to allow water to flow therethrough . the inside diameter of the filter core 54 is slightly larger than the outer diameter of the sliding valve tube 52 , so that the sliding valve tube 52 can slide up and down within the filter core 54 . it is preferred that an open - cell foam pre - filter 68 be installed over the filter core 54 . the valve 10 is assembled by mounting the drain 26 through the aquarium bottom 28 so that the standpipe 56 extends vertically upward , placing the valve guide 54 ( i . e . the filter core 54 ) within the upper inlet portion 62 on the drain 26 , placing the foam pre - filter 68 around the valve guide 54 , and installing the integral float and sliding valve tube 52 so that the sliding valve tube 52 slides vertically within the valve guide 54 . the sliding valve tube 52 extends downward from the float 50 along a vertical axis . the sliding valve tube 52 preferably has a circular cross section . referring in particular to fig4 and 5 , water flows into the valve 10 , as depicted by arrow 72 , in the space ( i . e . valve inlet ) provided between a lower edge 70 of the sliding valve tube 52 and an upper edge 74 of the upper inlet portion 62 of the drain 26 . water flow into the valve 10 is metered in proportion to the size of the valve inlet between the lower edge 70 of the slide tube 52 and the upper edge 74 of the drain 26 . preferably , the lower edge 70 of the slide tube 52 is cut so that water flow into the valve 10 is metered linearly with respect to physical location of the sliding valve tube 52 in relation to the drain 56 . the lower edge 70 of the sliding valve tube 52 preferably lies in a plane that intersects the vertical axis of the sliding valve tube 52 at an angle of approximately 20 - 30 °, and preferably approximately 25 ° in the embodiment shown in the drawings . the upper ledge 74 of the upper inlet portion 62 of the drain 26 preferably lies in a plane that is perpendicular to the vertical axis of the sliding valve tube 52 . still referring to fig4 and 5 , once water in the filter intake region 18 of the aquarium 12 reaches the height of the upper edge 74 of the drain 26 , water flows into the valve 10 through the valve inlet between the lower edge 70 of the sliding valve tube 52 and the upper edge 74 of the drain 26 . within the valve 10 , water falls due to the force of gravity through the drain 56 as depicted by arrow 76 . a restricted amount of water flow is allowed through the valve 10 when the valve 10 is closed so that the water level surrounding the valve 10 ( i . e ., the water level in the filter intake region 18 of the aquarium tank 12 ) rises gradually above the level of the upper edge 74 of the drain 26 , and gradually above the valve inlet between the lower edge 70 of the sliding valve tube 52 and the upper edge 74 of the upper inlet portion 62 of the drain 26 . because the upper edge 74 of the drain 26 and the valve inlet are submerged , gurgling sounds generated by the flow of water with entrained air through the aquarium drain 26 are muffled . the water level in the filter intake region 18 continues to rise gradually with the valve 10 in the closed position until the water level in the filter intake region 18 rises to a buoyancy water line 78 on the float 50 . as the water level in the filter intake region 18 continues to rise , the force of buoyancy pushes the float 50 vertically upward , thereby sliding the sliding valve tube 52 vertically upward within the valve guide 54 . as the float 50 moves upward , the flow area of the valve inlet between the lower edge 70 of the sliding valve tube 52 and the upper edge 74 of the upper inlet portion 62 of the drain 26 increases , which allows a greater flow rate of water to flow through the drain 26 . as mentioned above , the lower edge 70 of the slide tube 52 is configured so that more water is allowed to flow to the valve 10 as the float continues to raise the slide valve . the diameter of the drain 26 should be chosen to provide a flow rate in excess of the maximum return flow rate from pump 36 through return line 38 and return tube 40 which discharges into the main portion 16 of the aquarium 12 . in this manner , the float actuated water level control valve 10 can open and close as necessary to maintain the water level within the filter intake region at a preferred water level corresponding to the buoyancy water line 78 on the float . in the embodiment of the invention shown in the drawings , assuming that the diameter of the slide valve tube 52 is approximately one inch , the vertical stroke of the slide tube is between 11 / 2 inches and 21 / 4 inches . therefore , the valve 10 operates to maintain the water level in the filter intake region 18 under steady state operating conditions within a range of 1 to 11 / 2 inches of the preferred water level , i . e . distance &# 34 ; d &# 34 ; in fig2 . note that the water level &# 34 ; d &# 34 ; within the filter intake region 18 is substantially higher than the height of the upper edge 74 of the inlet portion 62 on the drain standpipe 56 . thus , water spilling from the main region 16 of the aquarium over the top end 20 of the overflow wall 14 into filter intake region 18 has a shorter distance to fall before splashing when using the valve 10 . the shorter distance &# 34 ; d &# 34 ; between the top end 20 of the overflow wall 14 and the water level within the filter intake region 18 reduces the loudness of splashing noises . in the preferred embodiment of the invention , the diameter of the float 50 is substantially larger than the diameter of the sliding tube 52 , which allows the float 50 to be more vertically compact . however , to carry out the invention , it is not necessary that the diameter of the float 50 be greater than the diameter of the sliding valve tube 52 . the inner volume 80 of the float 50 is coextensive with the inner volume 82 of the sliding valve tube 52 . note that during steady state operation of the overflow filter system the lower edge 70 of the sliding valve tube 52 opens above a dynamic flow of water flowing into and through the drain 26 , arrows 72 and 76 . however , when the overflow filter system is idle , the water level in the filter intake region 18 may be above the level of the upper end 74 of the drain 26 . upon system startup , water standing within the sliding valve tube 56 drains contemporaneously with a restricted amount of water flowing through the valve inlet between the bottom edge 70 of the sliding valve tube 52 and the upper edge 74 of the drain 26 . the float 50 contains a vacuum relief hole 84 at a location above the buoyancy water line 78 . the primary purpose of the vacuum relief hole 84 is to allow standing water within the sliding valve tube 52 to drain upon system startup without creating vacuum or suction pressures within the coextensive volumes 80 within the float 50 and 82 within the sliding valve tube 52 . absent a vacuum relief hole 84 , buoyancy forces on the float 50 must overcome suction created within the volumes 80 and 82 , thereby rendering the valve 10 unstable . an elongated muffling tube 86 is inserted through the pressure vacuum relief hole 84 in the float 50 . without the elongated muffling tube 86 , a direct acoustic path would from the drain 26 through the valve 10 and through the vacuum relief hole 84 in the float 50 to the surrounding environment . the purpose of the elongated muffling tube 86 is to attenuate sound passing through the vacuum relief hole 84 in the float 50 , thereby maintaining one of the principal advantages of the valve 10 as used in an aquarium overflow filter system which is to reduce the propagation of gurgling noises to the surrounding environment . although the invention has been described as used in conjunction with an aquarium overflow system , those skilled in the art will recognize that a float actuated water level control valve 10 as disclosed herein , may be useful to control water levels within other types of reservoirs with drains . in addition , is recognized that various equivalents , modifications , and equivalents of the invention may be possible in accordance with the true spirit of the invention . such modifications , alternatives and equivalents should be considered to fall within the scope of the following claims .